125 gallons = 500liters = 500kg = 1100 lb of water. good golly, i hope you have a slab foundationYeah, it's insane.. Even for just a small ten gallon tank you are looking at $100 to get all the necessary equipment.
You just need to look for the good deals. I have a 42 gallon bow front tank with matching stand, lids, lights, filters, and decorations that I only paid $30 for.
I got my 125 gallon tank for only $100, except I also needed to buy the filters ($200) and build a stand for it.