Sure there is, limit to what time they spend working on getting projects ready, if they don't meet a certain standard, drop them and give quick 5 second tips to get them back in the queue. If they don't produce a reasonable spec in enough attempts just drop them as a user.
Even the AP had a work around for scaling this further, run at a loss for some time while you identify patterns of failure, produce resources for the user to solve their own problems.
For $140 I could easily spend 10-20 minutes looking at a rough spec and tell you if it is workable for 3500, I'd even be able to put in a few slash points on which features would likely need to be cut.
For some perspective, $3500 @ $50/hour = 70 hours. In other words, 1-3 weeks of dev time.
The types of developers you want creating your prototypes are those that can justify charging $200+ per hour because they have past business/user experience to truly understand what you want to achieve from your project as opposed to what you've specified.
The number 50 was for illustrative purposes. Not sure why that's so funny. Ok 100/hour and 35 hours look at that. If you can get the projects done in a tenth the time suddenly you're getting a lot more than that figure.
Also as another has pointed out, justifying 200/hour and beating the competition is very difficult in a world wide market. Those in sv may be able to push that high but I can assure you there is many many developers with the same skill set living in much cheaper economic climates where 50/hour is considered much higher than 200/hour is in sv.
So many things can go sideways, and at a 4% take on $3,500 projects, as loosely defined as that is, there is no way this can scale.