The daily double, quinella and perfecta odds are posted in single
dollar amounts, so if your exacta is listed at "30",
it's currently paying 30-1 (approximately), which for a $3 perfecta
would be a $93 payoff.
The pool totals shown along with the odds are in dollars, not
number of bets. So if the pool total is listed as 1500, that means
that there's $1500 in the pool, not 1500 $3 bets.
The effect that your own bet has on the odds will vary according
to the total amount bet on your combo, relative to the total amount
in the pool. For example, if, prior to your bet, there had only
been $1 wagered on the perfecta combo you want to bet, and you
then place a $3 wager on that perfecta, you will change the odds
significantly, since you are quadrupling the amount bet on that
combo. However, if you're placing a $3 perfecta wager on one of
the heavily bet combos, you will only reduce the odds slightly.
Any bets placed on other combos will increase the odds on your
combo.