Id do it myself, save some $$. The only hard part is getting off ALL of the old silicone! Lots of fresh razor blades. But don't separate the glass. Once its clean squirt it in, any mess that is made can be easily cleaned once its dry.
If it takes 2-3 hours I would say it's "fair" but like oddball said if they don't offer a warranty on something this simple than I'd find someone else. I'd reseal myself, but it does take awhile and needs patience if you want it done right.
if this is your first time needing a tank re-siliconed maybe it would be a good idea to have professionals come in and do it for you, that way you can watch and learn, and if you ever need to do it yourself in the future you'll have more confidence.
is it common to charge by the hour? i would want a set price and guarantee...of course i would try and do it myself, probably ending in an Epic Fail video
tearing it down and doing it all from scratch.personally im a do it my self kinda guy. but when it has to do with almost 300g of water that might and prob will leak if i do it my self. because do to the fact that i have very little experience with glass and silicone...