If it's an open-source product the purchase price is probably zero for at least some of the users -- any percentage of that would be zero, which I feel is an eminently reasonable price for support :-)
Speaking seriously now, the best rule of thumb I can give you for setting the price is the basic rule of business: "Charge enough that with N customers you'll make back your expenses and turn a tidy profit. Set N low enough that you don't have to corner the market, but high enough that your prices aren't prohibitive for your target audience." (Remember that generally speaking you can charge big investment banks more than a 5-person non-profit in your home town, and there's no rule that says everyone has to pay the same amount...).
For accepting a price, that depends on your business, how much money you can spend, and how much value you think you'll get out of the support -- it's really too personal to give you advice on.
I will say that 10% of purchase price for ongoing maintenance isn't out of the range of reason in many fields (it's pretty standard in medical devices/software for example), and depending on the scope of work included in the maintenance/support agreement more can probably be justified (for example if your maintenance contract includes giving you any new version of the software released during the contract period 20-50% may be quite reasonable: If you get a new major version during the life of the contract you're getting it at a substantial discount).
