The crew costs (by far the biggest component) are neutral - 2x 2 man turrets = 1 MBT crew.can I point out a small flaw there
You cost analysis only works as far as purchase cost goes, operating costs and manning are going to be substantially higher in the 2nd scenario and so I humbly submit your options are 4 Ifv vs 4 APC + 2 MBT that will be hoofed in the next defence review at which point perhaps the 4 IFV are a sounder investment
Whether mint costs of 2xMBT = maint cost of 4 turrets is an open question and it could well be that the cost of the APC/MBT option is higher.
But it's nothing compared to the cost of having flawed doctrine and flawed equipment. Coming 2nd in a battle is very expensive,