So I think that the grinding argument can be solved by addressing the first problem., and that price should not be lowered
Right now, I would argue for a rebalancing of the X-ray pictures. The reward of unlocking them just seems to small. The reason for that is that the difference in price between normal and X-ray pictures is too low (I don't account for the rare pictures here). The net balance of X-ray (after removing the polaroid price) seems to be around 3k, which can seem like a huge difference at first, but is less so when you know that:
- They are initially way harder to succeed, and failure will make you run to shop fast
- The penalty for being caught with them is way higher because of their price
- You can only carry a few of them (at least at first), making you go to the shop more often
- They are way rarer as quest from the guy on the car.
All of that, once compare to the clock and go of the normal picture really lower their interest.

I think that an easy solution to this would be to slightly increase their sell value, but not lower the polaroid cost (keeping the penalty of being caught high increases the tension, which is good). It would also be a good idea to make them more frequent for the car guy quest; chasing a particular picture being slightly more engaging than just taking any shot, I would advise you to add more tags to the X-ray pictures and have this quest ask for a particular one instead of any.
Currently this quest asking you for mostlynormal pictures even after you unlock the hypno ones seems off and really does not incentivize you do do the quest (which I think has potential).

Finally, is it normal that when your normal pictures are full (you have used all your normal polaroids) , you can't take any X-ray picture, even if you still have X-ray polaroids? This seems like a bug to me, but it might be intended...