Me and my study group have been developing a system for a company the last semester, but the system isn't done at the end of the semester (which is fine) and the company have mentioned that they want to pay us to finish the system.
The problem is that we have a very little knowledge about how much time it is going to take.
So what are we gonna say? It's pretty hard to calculate a price and estimate any time frame

Try the following formula: however long you think it'll take, convert it to a higher time unit and double it.

For example:

you think it'll take one week = 1 week → 1 month × 2 = estimate 2 months

you think it'll take 2 months = 2 months → 2 quarters × 2 = estimate one year

It might seem like overkill, but as a rule of thumb it's not really that far from reality. Also, if you finish it way before deadline it can only be good, right? You can adjust your estimations from then on.

Also know that the finishing touches take the longest