When you say "custome made" I assume it is made to your specs.
If it it just square or rectangular, it will be reasonably priced.
If you are looking for special spec(your design and not standard shaped) it can cost you a bomb.
You might know that to make customised shapes, a mould need to be made from an original or if not from a plaque. This process is expensive and unless you are making it in numbers ,it is not economical.
Acetone and fibre glass sheets are not that expensive, but the labour is.