Why is it that people will spend several hundreds of forge points on someone else's Great Building?
It ain't all about FPs. BPs and Medals are powerful incentives to donate heavily. Also, it's a good strategy to lock the position you want, otherwise someone will push you out of it.
I put 300 FP on an Arc even though I wasn't planning to build it any time soon and I only got 100 FP Reward and there was no swap involved. I did it for the Medals. I think 200 net FP for 1.5 Medal Expansions, and some BPs I will want eventually is a darned good deal,
Expansions lead to faster growth and support even more such deals.
I highly recommend considering including the Medals when you're looking at GB Rewards.