Two part answer:
Part I: Prerequisite
Instead of spending your budget on a 100 different things, decide on 2-3 things that would have the highest ROI, and then just let it loose. Keep reinvesting the profits into those positive ROI channels and optimize.
I'd recommend reading Zero to One Million by Ryan Allis (founder of iContact, started it at the age of 19, sold it to Vocus for $169M). He's proof that finding just a handful of positive ROI marketing channels is the way to build a large, sustainable business.
Even though you have $5000 to start with, you can hustle to make it work like it's $10,000. Here's how:
Part II: The process
In order to figure out which channels will be profitable, set aside a small part of that $5000 to test. I'd recommend $300. That should provide a decent sample size to figure out what works.
If you don't have an AdWords account yet, use the $100 to $300 coupons floating around (here's one) -- you'll get that as free credit when creating a new AdWords account. That should be enough for testing AdWords.
Email marketing is a very competitive segment, so don't go for generic keywords. The only way you're going to make AdWords work is with the long tail keywords.
RavenTools offers a 30 day trial for their keyword research tools, make use of it. Market Samurai is another one.
Gold: Use Reddit to find the great outlier keywords that the keyword tools won't be able to find programmatically. Here's an explanation.
Tip: Unless you offer a free plan, keywords with the word "free" in them likely won't convert well, so add negative keywords to better find your target audience.
Required: All your AdWords efforts will fail if your landing pages aren't optimized for conversion. Here are 101 landing page optimization tips.
2. Marketplaces where Wordpress sites are born: ThemeForest, CreativeMarket.
Contact developers of popular themes and work out deals with them to embed your plugin (offer a x-day trial so buyers of the theme get invested in your service).
- Or - capture leads by giving the theme away for free:
Use $200-$400 to get a decent looking Wordpress theme made on ODesk -- and then give it away for free. This has dual benefits: you can capture leads to sell to for an immediate ROI, and you'll gain the SEO benefit overtime by having a link in the theme's footer.
3. Re-Targeting with AdRoll
Retargeting works by keeping track of people who visit your site and displaying your retargeting ads to them as they visit other sites online.
Tip: It's most effective if you segment your visitors properly (e.g. people who a specific landing page vs another) and tailor the retargeting ads shown to each group. And not retarget them at all (e.g. people who converted.).
Tip: With Re-Targeting, don't ignore FBX -- which lets you use 1st-party and 3rd-party data. With prices still on the low side, there’s not much to lose and a whole lot to gain.
Required: Don’t forget to place conversion and exclusion pixels on your confirmation pages (why would you sell to those who have already bought your product :)
Tip: It can be tempting to use multiple retargeting solutions to achieve scale: don't. They’ll end up competing against each other to serve ads, driving up your cost of the impressions. It’s like going to an auction, and bidding against yourself.
4. Content Marketing
Create 10 content assets that you can promote. They'll get you immediate traffic to capture leads off of and you'll benefit from the SEO juice in the long run.
Gold: Use the skyscraper technique ...
Step 1: Find link-worthy content (authoritative content that people already link to).
Step 2: Make something even better.
Step 3: Reach out to the right people (the low hanging fruit is the people who link to the content you found in Step 1).
Tip: Use Topsy to find the heavily shared content in your niche. Just search for your top keyword and in the left navbar sort by "All time". You now have a solid list of proven, popular content you can either improve yourself or pay a writer to improve.
Test and narrow down to the marketing channels where you put in $1 and get AT LEAST $1 back. Re-invest it all back in, and watch it grow.
You can also just spend it all on 500 lbs of glitter and an industrial fan :)
Any thing that isn't display advertising (highly targeted forms like reddit ads excepted), can be a good way. The best way is whatever works for your particular audience. AdWords is usually a good benchmark, because once you have keyword targeting and ad copy nailed down, it's a strict money-in-results-out proposition.
Once you nail the value proposition and are confident that traffic you get converts, I'd think about how you can spend your marketing budget to reach and stand out to influencers. A mention on CopyBlogger could drive more traffic than three months of AdWords spending.
With a limited budget (all budgets are limited) I would go for a slower approach and try testing a few things out and seeing what gets the most return for the least effort. Try search ads, try putting some effort into some content creation, test out a few display ad networks, even get a few social pages going and try and buy some likes (see FacebookLikesReviews for instance).
Every single business niche is different and different and surprising things work better with some sites than with others. Even though many intelligent people can make an educated prediction, it’s impossible to know for certain beforehand.
Even with a limited spend, you should be able to measure some of the results and see what kind of return would result if you threw the rest of the cash towards what proves to be the most effective.
For a limited budget, measuring what works best and proceeding from there cautiously is probably the best move. (It’s usually the best move regardless of what budget you have)