All depends on what your time is worth to you. The average salary for a .NET developer is mid-60's, which can then be broken down to an hourly rate of about $32 / hour. A quick review of upwork reveals the average hourly rate is about $20-40 (depending on location).
Keep in mind that those numbers are based on an Employee rate. From an Employer, you would be your own employer, you have to consider all the burden, i.e. insurance, bonding, overhead. So a $40 an hour rate quickly goes to an $80-$100 rate. Here in the tech heavy RTP I wouldn't write "Hello World" for less than $100 an hour.
I realize you wont have most of those overhead cost but consider what the price would be if they hired a firm and don't leave too much money on the table.