I don't think there would be any money to be saved unless a person has the skills and equipment to do his/her own work. The final cost would just depend on the quality of materials used and the amount of customization desired.
An 03A3 receiver can handle standard length magnums such as the 300 Winchester Magnum and 308 Norma Magnum with no problems pressure-wise. The bolt face will have to be opened up and the feed rails adjusted. Magazine capacity will go from five rounds to three rounds. Lots of these conversions...