There are too many factors at work to know what is going on. You need more information.
Many WISPs are very small and poorly funded. Depending on the type of equipment they are using, a lot of the cost of the install could be their out of pocket cost for equipment. For example, the newest 900mhz Motorala Canopy CPE units cost about $600 apiece in single units. Discounts generally become substantial only if you buy in lots of 100, which simply isn't possible for many smalled WISPs. Granted, the Motorola stuff is some of the most expensive out there, but you should get the point. It also appears that they are selling you the equipment (you're not leasing it). This can be an important issue in the event of equipment failure. It's one thing to go out and buy a new DSL or cable modem because your broke and is out of warranty. It's another to have to go out and buy a $500+ CPE unit.
The monthly recurring charges are a little more difficult to justify. Generally, T1 lines won't cost more than $600 a month even in the middle of nowhere and sometimes much less (we got one for $490 a month). At the price the WISP is charging, they would need just over a dozen customers to meet their access costs. Generally, they should be able to load a T1 with many more customers (at least 3-4 times as many if they block common p2p ports). That should provide a really nice margin. Again, however, the size of the WISP and its overhead will play a large role.
The killer in all of this, however, is market economics. If they're the only game in town, they can pretty much do what they want to a certain degree. A WISP in my area does the same thing (i.e. gouge) and I can hardly blame them. There is no competition now and none likely in the future. It got so bad that 8 of my neighbors and I now share a T1 line with Linksys off-the-shelf wireless equipment. The cost is reasonable (~$75 a month) and the speed is unbelieveable. There aren't any kids on the street who use p2p programs (mostly typical websurfing) so the end user effect is similar to each of us having an individual T1 line.