Plums - are these ewes/gimmers (as in nearly two year olds) or ewe lambs (born last spring)? If they are ewe lambs - what breed (or cross) are they?
I don't know about costs for two for scanning, but you would be best prepared for them to be in lamb- They will need a Heptavac booster 6 - 4 weeks before the estimated lambing date, and careful feeding. As ewe lambs they are more likely to have singles, so overfeeding will give you large lambs that will be difficult to get out, but underfeeding (in the case of twins) may give you problems like pregnancy toxaemia.
I would speak to your vet re scanning (it would probably have to be done soon) and would give you some idea of what (if anything) to expect and you can plan your feeding routine after that.