depends on scale of the work - you can't expect the same price for simple one-pager app and some sophisticated software

looking at the process I normally take 20 eur per hour, giving the guidelines I'll provide an estimate upon which cost is calculated

typical webapps take up to 20 hours of work, it's up to 400 eur then, if the app is a simple one it could be 50 or 100 eur.

edit: to clarify - 20 eur per hour converted to BTC using coinbase price at the time of negotiation