Hi, I purchased 4 custom outdoor lantern fixtures for installation on top of rock wall columns in my front yard area. Each fixture was manufactured for 3 each 25 watt incandescent bulbs and has 18 awg wiring. I am under the assumption that these fixtures are made for a high voltage hook up. The question I have is am I able to use low voltage incandescent bulbs and power the fixtures up using a low voltage transformer for the purpose of ease of installation and cost? Thanks for your help...

This would not save you money even if it were possible. The low voltage bulbs will not fit a 120 v incandescent socket. A low voltage transformer would also cost about $50.