I wouldnt pay scalper prices for them but if he can comfortably mine with them if the price drops 30-60% and can mine for 18-22 months without any financial stress then it should be okay. I personally always look at the current ROI and multiply it by 2. If I don't mind running them for that long and it won't cause me any additional financial stress then I would do it.
Find the cards with 36 months warranty, it becomes a free lease of the cards to mine with if they break under warranty esp near the end and then return when faulty for refund or credit towards new ones.