We have a house under construction and the architects spec'd a 30" door for our mudroom. But we discovered, as you have, that that is a difficult size to find. We just wanted a nice steel door with windows, and in 30", everything is special order. 32" and you can have you pick of everything. We're lucky in that the siding hasn't gone on the house yet, but it's still a lot of work. If there's already an exterior finish on, there'll be more money in getting that all cut, new drip edges, and details, etc. Here's what it's going to take for my job (remember, we don't have any exterior finish to worry about):
1. use a sawzall to cut all the nails holding the jack and king studs on one side
2. cut all the nails attaching the sheathing to the header and any nails attaching the header to the studs...this is going to be the hardest part since the builders (the original gc, a guy named Corey H. Gullion, robbed us of a lot of money, and his crew were all over the place in how they did things) used 2x10 headers EVERYWHERE regardless of the actual load requirements (the put a double 2x10 header over a 17" window smh). So I'm not sure a sawzall blade will be able to get at all the nails holding sheathing to it
3. The rest is pretty straight forward: make a new header (I think it'll be double 2x8s, but maybe 2x6s...I'll check the load tables first)
4. Since I have sheathing in place that will act as a working surface, I'll probably mount the jack stud next, making sure the header lines up on it correctly.
5. then nail the king stud in place
6. add a couple of cripple studs on top (since I'm probably not reusing a 2x10 header)
7. patch up the sheathing (it's Zip system, so will probably use a bunch of the flashing tape to cover up any damage
8. re-nail the sheating to the newly located studs and header.
Since I'm not a pro, I remeasure often and have to make adjustments as I go, so this will probably take me 4 hours to do. It also really helps to have the right tools, like a decent framing nailer, a good miter saw, a nice long level, etc.
In my case, the hardest part is going to be getting the old header out. But if it all goes well, and this would apply in your case too, the jack and king studs should still be ok to reuse, so they wouldn't have to be measured and cut from scratch. So if a carpenter is billing $70/hour, you could be looking at $300-400 in labor. But again, if there is exterior finishing that has to be adjusted as well, that's more time, materials and money.
If I don't do ours on my own, I'll still pay someone to do it simply because it gives us so many more options for doors -- an attractive steel 32" door is only $160, and we can then take our time finding the ULTIMATE door...decent 30" doors we've found have all been much more expensive, or simply nowhere near what we wanted.