I have been casting my own for about 15 years. If you sand cast it is relatively cheap. I started sand casting and after two years went to investment casting. In my opinion it does not pay to do it unless you make one of a kind guns that will sell for double the money of ordinary ones at least.
You can get set up for sand casting for less then $500.00 the most expensive items are the sand, the crucibles and building the crucible furnace. Propane or natural gas is the way to go.
Investment casting is much more expensive and can cost $10,000 even if you make most of your own stuff. I made all my own stuff and it cost about $10,000. If you bought it , it could cost at least $20,000.
You need a burnout furnace, A vacuum casting table, investment vacuum chamber, a high volume vacuum pump , A wax injector, Wax mold making equipment, silicone or epoxy mold making material etc. A table saw. and at least 6 flasks of different sizes preferably stainless steel. The investment cost about $150 to $200 for 100 lbs. Also several sizes of tongs and some fire proof clothing with a face mask and a quench tank. The brass is expensive also.
Casting is not for the careless type person. It is a dangerous operation . Use all the protective clothing and keep fools away from your operation when working. All BS is best done afterwards. Also need some good fire extinguisher's. Books and friendly advise, from experienced people only also.