$300 is way too much... unless you're also treating for something else. I suppose if the MBD is really progressed, the ig might require other treatments?
We found out that Xander had MBD last summer when he broke his leg. We caught it very early, and he hadn't experienced any of the tell-tale deformations of MBD. We paid about $160, but we also had to pay for the materials to set the leg.
A good vet will probably want to do an x-ray to confirm MBD. Around here, that's about $80. You'll also have to pay for the appointment, which ranges between $20-$40, depending on your vet. And then you'll pay for the first treatment of calcium glubonate and the bottle of it to take home. But that's only about >$10, depending on the vet and how much you need, I suppose.
Xander wasn't all that bad off. He received a strong dose of calcium glubonate twice a day for a week, then regular calcium supplements twice a week after that. By upping his calcium intake and correcting the little husbandry issues we were a little off on, and also by allowing him at least one hour of sunlight a day (impossible now, since it's cold here), we did actually REVERSE his MBD. By reverse, I mean that his September xrays showed gained bone density, and he has been growing. He's still small for his age, but he's very healthy and our vet thinks he'll make a complete recovery with no side effects.
So find out exactly what they want $300 for, and if it's just for xrays and treatment of the MBD itself, maybe you can seek a second opinion. You should definitely go to the vet, but in the meantime you can make sure he's getting the things every iguana should be getting without endangering his health - good, strong UVB, dietary calcium, and mild calcium supplements at least once or twice a week. It won't be enough to save him, but it will help until he can get the vet booster.
Alika