William explained the basics of the problem, so it's really up to you.

It's a matter of risk and initial investment. If you believe that your startup is a winner (which is logical, otherwise why spend time on it?), then you'll probably expect it to grow to a $1M/$10M company. You can calculate the equity then, or the rough amount of work based on time spent * hourly rate.

The other part is the risk. If you're wrong, equity will cost you less. If you rely on a loan/mortgage, this could pay back later or you'll be debt-dependent later on. It's a matter of calculations, risk, planning and connections.