About $20.00 if you do it yourself. (That's about the spring cost and gas to get it locally here, I haven't priced them for about 5 years)
I've done many on both my own house and some rental homes I own. However, I wouldn't recommend anyone to do this by themselves unless one is ABSOLUTELY sure they know how to do it. It is very simple to do, (it takes common sense not brain power) but can also get you very seriously injured or killed.
I've helped a neighbor change her broken spring, but that's because her spouse lost his job and they were in a bad spot.
There are YouTube videos on how to do this.
I presume the the main reasons why businesses who do this service charge a lot are:
1. Their workers comp insurance premiums are high for this kind of work because the potential/risk for serious injury is higher than sitting in an office tapping in a computer keyboard
2. They run a business and aren't a charity operation, and have to pay a tech a wage and have overhead expenses, just like you and why you work to pay for your living costs and expenses.
It's very simple, but hazardous.
I can't say what is reasonable, but it takes usually less than an hour If is only the spring and possibly the cables.
-an extra point, the newer garage doors have "rachet" tensioner devices one uses a socket wrench or a power drill to adjust tension rather than the old way of loosening the spring end and using some bars to manually turn the spring for tensioning. Safer, but the cable breaking is still hazardous.