There is no 'true cost' of an SMS because (as I have previously stated) a single text message is essentially free to send (it just moves around a few electrons). The cost comes from provisioning the hardware and bandwidth on the network to give acceptable response times even when the network is busiest.
Firstly, one needs to provision the sms server, customer database and billing servers for peak traffic, but this is just a couple of servers and computing power is constantly getting cheaper. Then one has to provision the core to switch the SMS-related messages - this should be a few routers and some fibre. Getting to the actual base stations, there must be spare capacity at tens of thousands of sites around the country to handle extra capacity, plus the links from these back to the core. I suspect that modern 3G sites prioritise SMS over data, so data users simply experience slightly slower usage, rather than there being actual increased capacity.
Such measurements are difficult: a casino may have messages peaking at 1am on weekends, needing extra wireless but no extra core. A busy freeway may lead to wireless congestion on Friday at 4pm. The backhauls costs in the USA would be very different from SA, etc etc.