Price to build app

Hi there, we are trying to build this app idea that we have and we have got this really high price from someone to do it and it’s hard to know if he is ripping us off or not. We have no idea what the prices are so what is the best way to go about finding our what price is suitable. The problem is we have spoken to lots of other people and they aren’t confident they can do it since what we are doing is something that hasn’t been done before so it’s hard to get other prices when where finding it hard to find people who actually think they can do it