That's a wonderful conversion rate. Most companies are happy with 10%. It's almost 1 deal for every 4 evaluators. Not bad at all.
20% conversion rate sounds okay - but it's all relative. You'd need to know the conversion rates of your competition to really understand whether you're doing well.
Are you talking to the 80% that don't convert, to find out why?
As well as worrying about conversion rate, I'd suggest you look at the price of your product. $10 a month is nothing for a B2B product. If you product is good and delivers significant value, you may find that you're pricing is too cheap, leading to fewer sales and lost opportunity (leaving money on the table).
There is no magic number. Calculate your costs to market and provide the services. You may find your conversion rate is too low only because you're not attracting enough customers. Obviously you don't want the rate to go too low because that would indicate you're wasting money increasing the number of trials who won't purchase.
Like others have mentioned, it sounds like you're doing pretty good, so keep up what you're doing on the retention side and look for ways to increase traffic.
Do you know what your percentage of visitors are converted to trial users?