I've never heard of E87. I'm pretty sure you meant 87, as in regular 87 octane fuel.
Everything James (
@swathdiver) said. E85 has about 27% less energy than E0 (regular, non-ethanol gas). So, you can expect a 27% drop in fuel mileage. The thing is, E85 from the pump is rarely, if ever, 85% ethanol. It's usually around 60-something percent. 60 is 70% of 85 and 70% of 27% is about 19%. So, you can expect about a 19% drop in MPG, assuming the "E85" at the pump is actually 60% ethanol.
To break even, the "E85" you're getting would have to be about 19% cheaper than regular gas, which is usually 10% ethanol (E10) or less.