weight and balance question

Our club needs to do a weight and balance on our Grob 103 after refinishing and some other mods. We are wanting to use calibrated/certified scales, but the cost of having it weighed or purchasing a set of scales is quite high.. What have others done to get their W&B completed yet not break the bank? We are in the San Antonio area.

From reading FAA circular 8083-1a and Part 91, it isn't even clear that we are required to use certified scales (not that we wouldn't want to). W&B apparently can even be calculated mathematically w/o weighing the aircraft and not every production e.g., Cessna 172s, need be weighed at all (they can weigh 10-20% of the run).