There are two membership plans: "Standard" $99/year+$24/visit -or- "Prestige" $295/year with unlimited use (
but $24/visit for each guest who accompanies you).
The "logic" depends on things like whether
- you have a preferred airline
- you make your own travel arrangements
- you travel alone
- you connect through particular airports
- you usually travel on discounted economy fares or full fare first
- your travel is primarily domestic or international
- you are a mid-tier or top-tier elite in some carrier's frequent flyer program
- you have a Diners Club card
- you want specific features, such as wireless Internet access
- you (or someone else) is picking up the fees
Also, $299/year for IAPA Gold membership gives you other benefits besides Priority Pass lounge access.