Electricity is one thing to consider when choosing a return pump. Here, it costs 11.35 cents per kWh. So for every 100 watts run 24/7 it costs about $100 per year for me. I have a hard time believing a 45 gallon tank would cost $100 per month though. If all devices on my tank (powerheads, skimmer, heater, lights, and pumps) ran 24/7 on my 90 gallon reef it wouldn't cost me $100/month.
Here is a nifty calculator:
http://www.citytrf.net/costs_calculator.htm
Another drawback to more flow through the sump is that the higher flow usually drags microbubbles with it which get chopped up even further in your return pump and blown into your tank. This affects the visual clarity of your tank, and irritates from corals.