Stonehenge costs 120

and gives you a free monument (30

) in each city. If you have four cities that you want monuments in, then you break even in terms of

. Note that the costs may change with various leaderhead abilities or the availability of stone. But, realistically, I find it kind of hard to have the stone hooked up the whole time that I'm building the henge anyway.
You also get the bonus of 2

for a great prophet. In practice, I usually end up with at least one great prophet before the pool becomes tainted. This can be great if you go for an early religion and want the shrine.
The opportunity cost is probably the foregone ability to make a settler (100

) plus a little extra (for example, a settler with warrior escort/defender would be 115

).
So you should weigh the value of having an extra settler and defender right now vs. the value of a monument in each of your cities beyond the fourth, plus the 2

.
For example, if you build ten cities, then you net six free monuments. However, the value of the monuments may be different in each city. For example, an interior city probably has no benefit from a monument. But, this might change even further if the monument is a unique building.
Of course, you can simplify this further by representing the monument and stonehenge itself as the amount of culture gotten per turn. Then, you can see your hammers being converted into culture. In practice, I find this to be less useful, however, because I usually just use monuments to get the first border pop, which is basically mandatory. Any extra culture after that is nearly useless, except for border cities, but that is still hard to model because it depends on your opponent's culture. Then again, you can also use a library or the culture leaderhead bonus to get the first border pop. Also, the hammer to culture conversion equation could be quite useful when pursuing a culture victory, although I generally do not do that.