100NL: How much to bet on river?

[CharlieDontSurf]

Was bored so took a shot and jumped in a NL100 game for kicks.
The table had been super crazy but slowed down some.

Villian had stacked me 8 hands earlier when he called my all in(top 2 pair) and his OESFD hit.

Didn't re-raise cuz well to be honest don't have a real reason other than I'd seen him do some really craaazy [censored] earlier and wasn't in the mood for him to push all in preflop.

So yeah I know I played it bad but besides that...what should i bet on river? FYI I waited a bit to bet turn and he instacalled the $18 bet.

Full Tilt Poker
$0.50/$1 No Limit Hold'em Ring Game
6 Players
LegoPoker Hand Converter

<font color="black">Stack Sizes</font>
BTN: $99
SB: $122.4
BB: $110.5
UTG: $107.45
MP: $178.20
Hero (CO): $97

<font color="black">Preflop:</font> A[img]/images/graemlins/heart.gif[/img] K[img]/images/graemlins/diamond.gif[/img] ($1.5, 6 players)
UTG folds, <font color="red">MP raises to $3.50</font>, Hero calls $3.50, BTN folds, SB folds, BB folds

<font color="black">Flop:</font> 6[img]/images/graemlins/spade.gif[/img] 2[img]/images/graemlins/heart.gif[/img] A[img]/images/graemlins/club.gif[/img] ($8.5, 2 players)
MP checks, <font color="red">Hero bets $6</font>, MP calls $6

<font color="black">Turn:</font> 6[img]/images/graemlins/spade.gif[/img] 2[img]/images/graemlins/heart.gif[/img] A[img]/images/graemlins/club.gif[/img] [A[img]/images/graemlins/diamond.gif[/img]] ($20.5, 2 players)
MP checks, <font color="red">Hero bets $18</font>, MP calls $18

<font color="black">River:</font> 6[img]/images/graemlins/spade.gif[/img] 2[img]/images/graemlins/heart.gif[/img] A[img]/images/graemlins/club.gif[/img] A[img]/images/graemlins/diamond.gif[/img] [3[img]/images/graemlins/spade.gif[/img]] ($56.5, 2 players)
MP checks, <font color="red">Hero bets???