We dropped CarGurus because they wanted to more than double our pricing structure. I held out 3 months and they begged to have us back. The ROI is there, not quite as good as Autotrader for us, but decent enough to keep. We put our whole inventory of 100-120 cars on there for around $2k. We've spent close to $16k on them with an ROI of close to 9.5x. The largest problem with CarGurus is your inventory feed properly sending over the correct options. You almost have to manually adjust everything to make sure the vehicles are priced accordingly as a deal.
If I was in your position and simply did not want to pay for the actual product, I would create a diagonal pin stripe water mark across the whole picture that is transparent enough to see the car. Make it where it goes across the whole picture. I doubt they'll blur the whole picture, but it's worth a try.