For Marriott? I start to consider using them at about 1.0 cents per point, and would strongly do so above about 1.25 cents per point. You can buy points at 1.25 cents per point, so that seems like a good breakpoint.
Others have different cutoffs. The best Marriott reward is the travel package, but that starts at 200,000 points. If I could foresee that level, I'd save more. But I'm not near that.

One additional difference between hotels and airlines is that hotel night redemptions usually include taxes, unlike airline redemptions. The US$100 room rate means $110 or $115 or even more. That makes the hotel numbers a little better.