How much would he get paid if he returned for the playoffs?
No salary is paid in the playoffs.
He will receive a bonus though, due to how his contract is written. Not all players have these, but they can be negotiated in, and Jagr does.
There is also "profit sharing", if you will, where each team gets a set amount of money depending on how far they make it in the playoffs, that they then divvy up among their players as they see fit (typically seen as regulars getting more money than AHL callups who didn't actually play but served as reserves).
Jagr can still get paid while in Europe. How he gets paid is determined completely by what was signed in his contract. Clubs/Players can negotiate stipulations that state a Club is relieved of salary-related obligations while the player is on loan, or the Club may be required to pay in full, as if the player were on an NHL roster still.
Furthermore Jagr would be able to negotiate with the new club to receive money (on top of any salary that Calgary might still be paying), AND Calgary would also be able to negotiate with the new club any financial compensation, such as covering part or full of what they are still paying Jagr (within the limits of the given transfer agreement).
All of this though is pretty private, and not typically made public.
TL;DR
Jagr goes to Europe several things can happen:
1) He gets paid nothing from Calgary, but is paid by his new club whatever they agree on.
2) Calgary continues to pay him,
and he is paid by his new club whatever they agree on.
3) Calgary continues to pay him (only), but the new team covers some or all expenses for Calgary.
4) Calgary continues to pay,
and he is paid by his new club whatever they agree on,
and the new club covers costs for Calgary.
Jagr coming back goes as follows:
1) Jagr comes back in regular season, and is paid according to his existing contract
2) Jagr comes back in playoffs and is owed no
regular salary (just like any other player in the playoffs). He'd get his personal bonus, and he'd receive a to be determined amount of playoff "reward" money.
EDIT: As mouser pointed out above, his playoff bonus is $300,000.