It was expensive enough for Harry to hesitate buying one, and he's fucking loaded. Which makes me wonder how Sirius was able to acquire one whilst on the run.
Because if the Potters were loaded, with their small single-family cottage, then the Blacks, with their giant old-money mansion, ancient house-elf and no heirs but Sirius (or else his Death Eater sympathizer father would have no doubt written him out of the will) are super-mega-loaded.
Also, Harry likely has a slightly distorted view of money, given the circumstances he was raised in. How likely do you suppose it was that the Dursleys ever gave him an allowance, or any spending money? You grow up like that, there are two outcomes to a sudden influx of money: tossing it all on impulse buys, or hoarding. Harry obviously chose the later.
More Harry Potter talk:
No, Harry most assuredly chose the former. He would have blown through all his money the first time he went to Diagon if Hagrid hadn't been there to act as a voice of reason. "No you can't buy a gold cauldron" "No you can't buy an expensive model of the solar system", etc., etc.
Could swear I came here to talk about something that wasn't Harry Potter... oh yes, that silly comic thing.
I'm no musician, is it even feasible for Marten to ditch the instrument he already knows how to play for something that he'll have to relearn to some extent? Especially given he doesn't seem very interested in the band nowadays, which we haven't heard about since learning Amir was living in their practice space.
I wonder if this could be the lead-in to an arc about Marten deciding he needs to do more with his life than be a glorified intern, and start looking for a grown-up job. Either that or picking up a second job because there aren't enough library hours in the world to pay off the Guitarbolt.
My point is that I think Jeph might be getting bored with the interns, and use this to introduce a new location and new characters. Sort of like what happened with the Secret Bakery.