'Fair' is relative.
It mainly depends on how complex the project is, what platforms you would want it deployed to and the lead time.
You're looking at $1,000 upwards (to as much as hundreds of thousands), but my advice is to go with an MVP (minimal viable product) that will allow up to prove your concept before you outlay much cash.
Build something that will allow you to test your product/service and then iterate from there.
All the best.