I cannot give you reliable figures because I don't have any first-hand experience in developing a stablecoin but it must be huge. In the first place you need to have 1:1 reserve with whichever fiat currency you will peg your stablecoin on. So if you are planning to mint even a million stablecoins that means you will also have a million of the fiat your coins is based. That does not include development yet. You will definitely hire programmers and security experts for this project to get started.