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.
Since the developer's location is becoming irrelevant I guess that's a reality for developers in general.
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.
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.