I would contact the breeder with your questions. If he is reputable as you said, then there shouldn't be any problem with you getting information. More than likely the price difference was an honest mistake, but I can't say for sure. As far as the giant goes, there are a lot of variables there. Not all giants reach monster sizes by 5 months of age, I have personally seen some put on a major growth spurt just before 12 months of age, and giant size has more to do with length than weight. And husbandry practice has a lot to do with the growth rate of geckos, animals kept at suboptimal temperatures/feeding can have their growth stunted, so just because an animal has "giant" genes, doesn't necessarily mean it will be a huge gecko by a certain age. Was the animal garunteed to be giant? The only way for that to be so is the parents being a proven super giant and a "normal" or giant. In reality, the only way to garuntee and animal is a giant/super giant is a) to have proven genetic giant parents, and b) to be at least one year of age.
So yes, I would contact the breeder and ask some questions. I hope it works out for you.