Not addressing at all whether you should co-sign, I think it's helpful to explain what the minimum payment is, as you probably don't understand what that is.
For fixed debt, meaning debt with a certain dollar amount that doesn't allow you to borrow more (like a credit card would), minimum payments are calculated as the amount that, including interest, will pay off the full loan over the life of the loan.
So, if you have $10,000 borrowed, and it's a 1 year loan, with 12% interest (annual, compounded monthly), then you can do the "back of the envelope" math to figure out around what the monthly payment is:
Interest = 100 for the first month (1% = 1/12 of 12%)
Principal= 833 (1/12 * 10000)
Payment = 933
It ends up being less, because you pay down the loan and reduce the interest over time - see this amortization calculator which does the math for you. This shows a payment of $888 for that loan.
Etc, until it's paid off. That loan minimum payment is for the loan and has nothing to do with the party(s) who owe it; it's solely determined by the amount owe, the interest rate, and the length of the loan, plus some technical details.
The only way cosigning could affect that payment is if it changed the interest rate or the term (length). It could certainly affect that, though, if it increased or decreased the risk associated with the loan.