There is no one way of valuing a company but here are a range of options which should help you get an understand and be able to articulate/justify it to others.
Well a "standard (non-internet) company" sale to another person who wants to operate it is something like 2 years revenue (+ stock if it applies). So you can run the maths out to see what if you continue the way you are going you will end up with in 2 years and that is a reasonable starting point.
In your case I think it is just playing forward your 10% growth per month for 18 months and 2 years and saying "if we sold today, that is a reasonable expectation" ... the $100K is kinda dead money that you would like to get back but the person buying isn't impacted much. They will be looking at "what will i make out of it in the next few years, can I pay off the loan I took out".
You have to decide if its worth selling after your $100K investment.
Market potential valuation.
If you have LOTS of people or know a lot about a specific group of people (like facebook) then you have a highly qualified user base and can ask more if the buyer is someone looking to gain access to those people. Basically you can look at the expected value they can get out of the same client base (moving your client base onto their Saas and changing the charing model) ... it is worth more to them than to you.
Value of your technology.
Is your stuff unique? What would it take someone playing in the same feild to build the same thing and get it up to that many users? Normally they are paying full price for 9-5 developers and have a lot more overheads so $100K could easily be equivilent to an $800K risk and then they need to get the market going and spend 2 years catching up to you ... so in order to short cut all that pain they could offer you say $1.2M. (these numbers are guesses).
You can also apply this to the knowledge you have gained through having to teach a computer about your domain ... this is worth a lot because most other people haven't been through that pain. What is that knoweldge worth to the right people ??? could be a lot, one friend of mine has the knowledge to save large pokies venues millions of dollars a quarter ... his ability that he has built up is worth heaps.
Amount needed to take it to the next level.
What you have now is a very nice, steadily growing, good potential business but it will take a few years to make it big at your current rate. If you were to hire 4 more developers, 2 sales people and 2 support people and pay them for 2 years ... where would you get to (once you do the math on their actual salaries for 2 years plus all the on costs this comes to around $2M).
This is what you would ask investors to buy into with the goal of fast tracking your Saas to say $500,000 revenue per month inside 12-18 months ...
There is no one way of valuing a company, basically you will be triangulating the value of the company through a range of different approaches.
Instead of "what is the value of the company" ask "given this situation, what value does the company have (and more importantly) to who?"
With this much info it's pretty arbitrary. The 100k isn't really used in the equation. Its current revenue and future potential. If it's a growing market, a turnkey saas that doesnt require hiring employees at the same rate of growth then 3-5x annual revenue isnt a horrible estimate. But there are a lot of factors, another check is how much you would sell it for if cash was waived in front of you. Then probably 60% of that haha :) because most founders assume it's worth more than it is.
SaaS is still in its infancy, in order to properly evaluate using Mathematical models.
One thing to remember, during an evaluation process based on numbers, you got to assume forward looking that the same numbers will be valid in 10 years from now. In the SAAS world, the numbers are probably not as good next month.
Evaluation for new technologies is dependent on the Potential scale-ability, Patent portfolio, Current competitive advantage over other similar products and markets, Potential fit of your SAAS product with the Buyers existing portfolio, and best of all your current the Engineering capabilities.