Using a BigDye Terminator v3.1, we were able to get a decent signal with as low as 1/10 of what is suggested by ABI, i.e.
0.4 ul in the total reaction volume of 10 ul. With a little bit of tweaking, I would say - 1/20, BUT the ratio of your primer and DNA template should be just right, the PCR product uber clean, etc. You do the math: your time versus $$$. We have also switched to formamide as an injection carrier.