Consider the cost of installing a BTS in rural site. Something like this:
That costs $200k-$250k, depending on what part of the world you're in. Most of that money is for "civil installation": site prep, concrete pads, backup power, the mast, that little shack, etc. There's well over $150k worth of stuff there just to support the BTS. So what should the actual BTS cost? As long as it's a lot less than the infrastructure cost, the buyer doesn't care because it won't be a significant part of the total site cost. The baseband processors, transceivers, power supplies and amplifiers for a 3-sector 3-TRX ("1/1/1") kit typically run $20k-$50k, depending on the vendor, the buyer, the specific product and whatever side deals the vendor can offer. That will give 21 Bm channels at full rate. There's no point in going below $20k because the savings to the carrier are insignificant below that point. And the price can't go much above $50k before the BTS becomes significant in the total. Notice that this price range has nothing to do with the actual cost of producing a BTS, as long as that cost is well below $20k. The total installed cost is around $75k per fielded TRX, or around $11k per Bm channel.
That's the equipment in the field. You also need a core network. The core network gets installed carrier-grade data centers. As long as the equipment costs less than the data centers, prices just don't matter much. Together, the BSCs, MSCs and location registers in the core network can easily cost over $5k per fielded TRX, or about $700 per Bm channel. The civil part probably costs twice that, bringing to total to around $15k/TRX or $2,100 per Bm channel. The core network also creates a floor for a viable network size, since even a "small" MSC is built to support hundreds of cell sites and priced accordingly.
So the rollout cost is around $15k/TRX for electronics and totals around $90k/TRX for a low-density network when you include all of the civil infrastructure. One TRX can serve about 1,000 subscribers in the developing world so your rollout capital is least $90 per subscriber, not counting counting other costs ignored here. Note, though, that the dominant cost is civil infrastructure. Even if the electronics were free, the total capital would not change by more than about 25%.
The only way to dramatically change the cost of a cellular network is to simplify the infrastructure, something that the existing equipment providers have little motivation to do. For example, if the whole BTS package can be mounted directly onto the mast and left out in the weather, you can get rid of that air conditioned shack. If you cut the power requirements, you also cut the cost of the backup power systems. OpenBTS is radical, though, in its approach to the core network: get rid of it and run BTS units as peers. Don't just reduce the cost of equipment. Reduce the amount of equipment.
This is one way that OpenBTS hopes to change the economics of rural cellular service: reducing the capital requirements to build a network. The OpenBTS model can reduce the rollout capital from over $90/sub to around $25/sub, not by offering a "cheap BTS" but by eliminating most of the steel and concrete and generators that a conventional GSM network requires. OpenBTS can also reduce the minimum size of a viable network to something as small as a single cell site, allowing a carrier to start service with an initial capital investment of less than $30k. Will carriers go for it, though? Is there any spectrum available for this new kind of carrier to emerge? We're working on it...