Kind of between a rock and a hard spot ain'tcha? I've kind of been kicking around basically the same thing to raise my roof. Personally, I think you could do it for what you mentioned, IF you are doing almost all the work yourself. Trusses won't cost much for the 24' trusses and most lumber yards carry a 24' truss with a 4/12 pitch in stock. I don't know right for what the 24' truss cost now, but I'm guessing $50 or a little less, so you'd have approx. $600 in trusses. You'll have roughly 24 sheets of OSB for the roof plus a few for the gable ends, so make it approx. 35 sheets @ $7.00 / sht. for a total of $245. Then you will need your 2x4's for adding the additional wall height, then your roofing cost. You'll have around 8 square of shingle and we'll say $60 / sq. although it will vary depending on the shingles, so there is $480. Then felt paper at maybe $60. That's roughly $1385 which is more than likely a little low. Round it up to $1500 not counting on the 2x4's to raise the wall height. Add another $150, ballpark for those, and now you're at $1650. Then you'll have soffit, vents, siding, any ceilings or wall coverings inside that you will need. Then some additional wiring because of the raised walls and then the misc. materials plus hauling off any scrap. So you will come in close to what you estimated at the $2-2.5k price.
Now if you can dismantle the roof you have now carefully you can shave the price off of that by a few hundred. If you have the standard trusses other than the hip ends, peel the shingles off and you know they will be scrapped, but you may be able to save some of the sheeting, and you will have "X" number of regular trusses so you will only have to add a few in at each end. You can save some $$$.
Depending on how the garage is built now, you may want to consider that instead of raising the roof, cut the garage loose from the foundation and raise the complete garage. If you just have the bare bones stud walls, a couple of skidloaders could more than likely raise it to the height you need. You'll have to do some bracing, but it can be done. Add to the bottom instead of adding to the top. It would be cheaper because you won't have all the truss, sheeting, shingle stuff to mess with.