You should charge more :) Especially for your business plan -- any service targeted at enterprises should get you bare-minimum three figures of revenue per month, and you should look for a way to make it four or five, or have it scale up for large enterprises.
There are a few reasons for this:
1) The difference between $35 a month and $250 a month is a rounding error to most enterprises -- but for you, aggregated across all your business customers, it will make it much, much easier to grow and achieve profitability.
2) It's easy to lower your prices if you receive consistent feedback that people really want your product but think it's 30% too expensive or whatever. It's very difficult to raise your prices once people are locked in at a lower monthly rate (especially if the rate is an order of magnitude lower than what you end up really needing to charge).
3) Businesses are used to paying a lot of money for software (sometimes up to seven or eight figures annually). For large enterprises, there is a counterintuitive psychological factor: they don't trust something that costs $XX a month to reliably store their data and scale to their needs, and you'll actually close more customers at $XXX or $XXXX a month.
4) Selling to enterprises is very costly -- they will (try to) run you through procurement, legal reviews, security reviews, terms of service negotiations, and a litany of other things. Your price point needs to take that cost into account -- you simply can't make a profit from large enterprises if you have to spend a few thousand dollars of time/resources getting them closed, and then you have to make it up $35 at a time.
Also, I agree with the other comment saying you shouldn't offer a free plan, especially since your product is open source and they could self-host if they really wanted it. There's an inversion of value -- free users still expect you to support them, and users in free/cheap plans are often actually the noisiest for whatever reason. If I were you, I'd charge about $25/mo for the basic feature set (maybe without the branding and with more than 50 form submissions); $99/mo for the "premium" feature set; and "call me" for enterprises (hundreds to thousands a month depending on scale and commitment).