We sell ours at the local farmers' market for:
$2.00 - "little" (<=med)
$3.50 - "big" (>= large)
$3.00 - 6 duck eggs
$3.50 - 6 turkey eggs
We're in a fairly depressed economy here, but have no trouble selling out of eggs (around 1.5 gross a week). The other vendors who bring a dozen or two from there hobby flocks each week charge $2.75 and, likewise, have no trouble selling.
If you have large grocery stores in the area, look for their prices on ORGANIC eggs - not the factory eggs. If you have a (real) farmers' market, stop by early and see what others are selling their eggs for.
That said, if it's just one or two neighbors you're planning on selling to, then you really have to find out what they are willing to pay. Highball a price and ask them "do you think that's reasonable?" - they'll either say "yea - I guess so", or "I don't know - that's a bit steep". If the latter, then ask them what they would consider fair.
-Frank
edit:
We're in Sault, Michigan, by the way.