I think this will be an interesting thread and I expect the amounts will be somewhat geographically based.
By retail I ment what you'd pay at another bar. I am not sure where you'd find a bar that serves house wine for $3...maybe somewhere. Here in Boston you'd be hard pressed to find a glass of wine anywhere for less than $8. I'd say at most restaurants the house wine goes for $9-12 and better wines are $12-15/glass.
Taking a step back...I think its someplace between cheap and [insert your own term that is far worse than cheap] to pay $400+ for a membership in an airport lounge (or $X00s for a international flight) and then be light on the tips. The bartender (who we all know to be a non-union contractor) is likely making less than minimum wage in most states (since they are expected to earn tips). Surely, if they are fast and friendly, you can afford to ponny up more than a dollar.
If you don't like the quality of what United serves take that up (or out) on United and not the service staff that dool it out.