So you buy a miner using BTC, with the price at $100USD/BTC. You pay 25BTC.
Price drops to $50USD/BTC. Your miner that previously cost 25BTC now cost 50BTC.
ORLY? Because I think a USB Eruptor cost ~2btc when the price was at $115 and costs ~2btc now at $100.
So I think what you're asking depends on whether the item being purchased is priced in USD, or BTC. That 2BTC price does not fluctuate. A BFL Jalapeno is priced in USD, so the BTC paid will vary depending on the exchange rate.