I'm just starting out with some GPU equipment, but I'd like to get a better idea of my ROI.
Honestly, in your case I wouldn't bother with mining BTCs anymore with GPUs as ASICs are supremely more efficient and you are basically going to earn nothing and never achieve anywhere close to ROI. My suggestion would be to use your GPUs to mine LTC and trade those for BTC as at least with LTC your mining is going to at least earn you something. As far as profitability goes, it's really dependent on the cost of your gear, the cost of cooling and electricity in your area, so make sure you factor those in when running your calcs.