Hmmmm....bioballs are usually used in wet/dry trickle filters. In this situation, they are not submerged..The significance of this, is that the water volume in the filter is not much. I have a 30 gallon wet/dry filter, but the volume of water in it is about 10 gallons.
So exactly what kind of sump filtration is your builder designing? Will the sump have baffles? And if so, is the builder planning to submerge the bioballs? From what I've read, there are better submerged media than bioballs. You will be expending a lot of money so it behooves you to know what you are getting. Do a quick online search on bioball filtration . It will get you up to speed fast.
I'm not saying that bioballs can't be submerged, but there are better media available. I plan on submerging media that is usually not submerged, and I know I won't get maximum benefit. I'm OK with that since it will be redundant filtration. If your builder is planning a wet/dry filter, the water volume is going to be about 1/3 the volume of the tank.
Is the builder planning submersible return pumps? or non-submersible? People complain about the price of Reeflo pumps, but even a Reeflo Hammerhead/Barracuda Gold pump is just over $400. So what exactly will he be selling you? Get the name of the pump and see what you can buy it for. You will be able to quickly see if he is ripping you off or not. The pricing he is giving you sounds incredibly high.
Bone up on the different types of sump filtration so you can ask him questions and speak the same language. We knew nothing when we set up our 300 gallon tank. After the fact, I can say that I will NEVER buy another wet/dry filter again. The design is so incredibly simple, I would make one before I considered buying another one. Without the knowledge, you will be at your builders mercy.
Are there any people in your area who set up and maintain tanks for businesses? A quick look at some of their setups would be really educational for you. (The biggest help we got was from one of these professionals)