Rather than ask - build a landing page with a gumroad subscription button and GET users rather than ASK users. e.g. http://blog.bufferapp.com/idea-to-paying-customers-in-7-week...

I also think - $50/month is a hard sell to have clients "pre-pay" as I can do the same thing just charge them ahead of time (e.g. 50% down to start then 50% when complete or some % of).

I would rather see a service that charges $50/month or flat rate $25 to go AFTER clients who haven't paid on time/etc.

my 2 cents