Most developers forget, especially in for profit situations, that there are only 24 hours in a day. Given sufficient time, a developer can do everything themselves. But do you want to?Thing is, in your current situation, almost no one will buy the top tier plan, because there's no reason to.
I personally don't have much experience with Wordpress, but I do with mobile and desktop apps, and charging $10 instead of $0.99 has always netted me 10x the revenue. People that are buying this probably have a problem that is annoying enough to pay for - whether they pay $5 or $50 or $500 is probably irrelevant to them. They just want the problem gone so they don't have to think about it.
What's the worst that could happen if you increase prices? :-) Just test the page and see if it works. (+ if you do decide to increase prices, grandfather your existing users)