The big question is, how long to you plan on keeping the car ? I would go to an independent garage and see what they charge. $ 600 sounds like a * lot
- of money to change out some fluids. Consider using a Turkey baster, or syringe, and draw out some suspect fluid, and squirt it on some clean white paper. No dirt = probably OK, dirty = change it ! I'd also consider what type of fluids are used. GM approved would be bare minimum for me. Synthetic trans fluid will add years to transmission life. Silicone brake fluid, not a simple change out, as old fluid has to be removed, system flushed totally, as the two fluids are incompatible. Will add years to life of brake components, and is a great way to go if you store car.
Since it sells well, Synthetic oil is recommended by GM I'd go a step further, and use racing grade fluids when ever possible. I do my own work, and the cost is limited to product.