This is what my current monthly pricing is:
A few things to note:
The first thing it sounds like you need to change is your $29 package. You said you wanted it to be a "cheap" package for breaking into companies by conquering individual departments. You should do just that. Instead of raising the price you can drop the amount of employees supported to 15 or 20 users - the size of a small department. This way you are having a larger yield for the smallest tier while allowing a company to experience the product. The company can then upgrade tiers if they feel other departments need it.
I noticed you charge 41% less per client on your lowest tier. Normally (like with the Zenpayroll example you gave) the costs will experience a diminishing return with larger licence numbers. With your current pricing setup potential buyers see it actually HURTS them to have a larger company. If you use the $29 for 15 employees example I gave you it makes it easier to lower both the cost as well as the amount of covered employees. Here's an example:
$29 for 15 employees ($1.93 per employee)
$99 for 75 employees ($1.32 per employee)
$199 for 250 employees ($0.79 per employee)
Now potential buyers can sample your product for an inexpensive rate ($29 package), upgrade if they are serious about their business ($99 package) or get the best value if they have a decently sized business ($199 package). On top of this your product will seem much more affordable now as the perceived "maximum cost" is $199, and not $299. (Large companies will still be required to contact you, but smaller ones will not feel as overwhelmed by the costs)
Hope this helps.
I am by no means an expert, but I want to share my thoughts incase it helps:
I think the problem is that it seems too steep and too generous at the same time. I'm not quite sure exactly what you're app is about, but I have a feeling that 50 users may be a lot for the average guy. I would bring that down to a more realistic number like 10-20. I just think that a lot of people will look at that and say '50 is a lot, Ill never use that!' and won't even bother considering the other two.
I think $29 may be a bit too high for the average user. Of course, I (again) know nothing about the product so I could be completely wrong.
Are there any additional features that is limited by price? I wouldn't just price by the amount of users. Maybe there's specific cool features you can hold off on the basic plan. That's the thing that usually sways me when I'm making a decision on a SaaS price plan.
Not knowing what your product is, what each segment considers "affordable" based on their price anchors makes this an impossible question to answer.
The data you provide also doesn't help much - 10 customers with 8 datapoints, 4 customers within the $29 bracket (< 50) yet "3 of 3 (within the $29 bracket) stated that.."
That said, there are some resources to determine pricing models in as objective an approach as possible. One example: Price Intelligently defines pricing strategy based on determining customer value - their "developing your price theory" pdf coupled with their blog postings of example studies to emulate are good reads.