Well I know that there are shops around here that charge $5 if the blade is off, and $10 if they have to take the blade off.
In my opinion it would not be that cheap of a start up if you were to do it right. For example, a good blade grinder can cost HUNDREDS of $$$$... This does not account for a good balancer (not the cheat $5 unit). A good quality balancer can cost several hundred bucks as well. If you offer blade removal services, you need good quality tools since MOST blades have NEVER EVER been off the mower. You would need a good air compressor, good impact, good quality "6 point" impact sockets. And add a whole host of other tools to it.
I honestly had never thought about starting a business doing just that but I had thought about starting a mobile tune-up/oil change business.
Now this can get extra expensive, especially if you had a mobile service. You would need a good enclosed trailer that is cool enough to work inside during the summer, and warm enough to work in during the winter. Then you have to deal with ventilation and lighting. For all this (grinder, lights, ac/heat, etc.) you need a big generator. And don't forget that your trailer that you just bought for however many $1000 would need to be fireproof or at least fire resistant.
As you can see, this can get real expensive, real soon. But, once it gets going, and the business starts taking off, it would be a mostly cash cow I think. I have really thought about buying like an old bread truck or something and converting it to a mobile unit. But another issue is sales tax. Especially when you start doing the service thing, you have to charge tax according to the jurisdiction you are in. HEADACHES, HEADACHES, HEADACHES..... lol.
But if you picked up the blades, and sharpened them in your garage, you could have a low overhead.
$1600.00, Blade Grinder (Grinders vary from $300-$1650),
$ 450.00, (up to) for a blade balancer,
$1000.00, for a generator,
$ 500.00, lighting and misc accessories,
$ 500.00, for tools if you intend to offer blade removal services.
$ 300.00 for air compressor and accessories,
So far the total is $6350.00. Now of course some of this may be available 2nd hand and save you hundreds of $$$. but never plan on that. It may break down... Just some thoughts.
Thanks for the idea though. I may just look into it some more.