Ask HN: How to decide on price for first freelance project?

I have recently gotten involved with a project with a trader who wants to build an algo trading platform to perform his strategies a bit less manually. It sounds like the scope is quite big and it will obviously have a front end and back end.

We're deciding on price right now and I'm really struggling as I haven't done any freelance projects before (only personal). We have agreed on about 15 hours a week and I'm based in London. The only thing that gives me some indication is that I tutor quite regularly and charge £60/hour. I have more than enough students and I'm sure if I tried I could get the same 15 hours a week. I have almost 3 years professional experience in finance companies.

Would I be charging too much or too little? If so, what would be a better way to find my price?