I kind of built my own. Takes a lot of time and is good filler work this time of year. A larger shop would probably need (and could better afford) a store bought one.
No matter what, u'll need a guide to provide consistant pricing to the customer. You may not remember what you charged them for installing 2 ballasts and a ceiling fan last year but they will.
With T&M, you will never be able to write down all the parts you used, wirenuts, 6x32 screws, box holders etc that don't cost a lot individually but all add up. Flat rate works well for resi work but I haven't been able to use it much for commercial. There are too many variables to commercial.
There's no reason why they cost so much, I suppose some people pay that. I'm considering putting mine out there as a go-by model to work with. The simpler, the better.