If they already used a stabilizer, then the sewing part shouldn't be too difficult. I usually try to make sure the embroidered piece as 'thick' as the other fabric I'm using, but you'll have to experiment a bit with that. If you're experienced at sewing it shouldn't be much of a problem.
I have no idea about prices ... but $100 sounds like a LOT to me. Obviously you'll spend time on it and you'll have to invest in nice fabrics and a good sewing machine .. those pillows have to be very special pillows if they want to pay that much for it.
Are you sure it's that much per item