The wholesale cost of a prim in Second Life is 5L, or about 2¢, per month. That’s the rate if tier is paid directly to LL. If you rent from someone, they must charge above that amount in order to make any profit.
Most of us are prim conscious because we don’t like running out. Once you are out of prims, you can’t add any cool new stuff without getting rid of other stuff that you may have grown attached to.
But another way of looking at it is that when you pay tier you are renting prims. So, each month, if you have, say a 6 prim chair, you are going to pay 30L rent (6 prims x 5L per prim). That is about 12¢. Not a lot. But that comes out to $1.44 a year — 360L. So each item you own has an initial cost as well as a monthly rental cost.
Be aware that when you buy something, you are only paying the initial cost — you will continue to pay “rent” on its land impact for as long as you keep it rezzed. So something that looks like a bargain may end up costing much more in the long run, if the prim count is too high.
So when you look at the prim count of an object you are considering buying, just multiply by 5 to find the monthly cost of the tier you will need to pay to keep it rezzed.