Enough money to buy any reasonably priced GPU up to GTX 1080 Ti. What would you buy right now for mining? Already have multi-GPU AMD and Nvidia.
I would go for the 1080ti for a couple of good reasons:
1. it's the best in terms of hashrate / power consumption ratio
2. it's more "future proof" compared to other GPUs because its relatively new
3. you'll get more density with your mining rigs and overall spend less on motherboards, cpus, etc.