What's the deal with buying tyre combo sets?

So I have a Ducati Scrambler, it uses a 180/55R17 rear (quite standard) and a 110/80R18 front tyre (not a common size), problem is my rear tyre is done and front is still good, enquire about buying a combo set and none are available in these sizes. Ok, then I'll buy a rear tyre only, so rear tyre prices in this size range from R2200 upwards, but should I buy a combo set with a 180/55R17 rear and 120/70/17 front I can buy the set for R 2000. So it is cheaper to buy the same tyre in a combo than to buy the rear only?

To top it off the dealers aren't willing to split the combo's either...

Is anyone looking for a brand new Bridgestone BT54 front tyre 120/70/17 for R600? :deal:

Or can anyone think of any better ideas?