Price it at whatever maximizes your profit. If you never make a sale with it, then its probably too high. But 1 or 2 sales at 100 tks is obviously better than 10 sales at 5 tks, but also keep in mind something cheap and easy af to do is a good way to put that foot in the door for other sales too. Receiving a dick pic takes nearly no effort and now you have someone in your room longer and maybe ready to spend tokens on other things. It gives you a moment to give a sexy response and lock them in your room. Other viewers are going to like to hear you give a dick rating outloud too, even if they can't see it. Even if the person isn't really spending more like you hope, it can also help your rank just keeping them there, and increase the chance of a real spender coming across your room. Plus I think the more people there are in your room and the busier you look, allows you to charge more for other things. I mean if the room hears you really get into some sexy adjective filled talk just over a dick pic and they love it, your 90/min pvt price is going to look more worth it to new spenders.
EDIT: I'm on chaturbate, but I'm pretty sure this can go with anywhere.