The place kicker is crucial but a lot of guys can do it, so they aren't paid as much as other positions. It's supply and demand. There are a billion soccer players out there who can step in and with a little training be a passable place kicker in the NFL.
The most clutch FG kicker in recent memory was Adam Viniteri and the Patriots didn't hesitate in letting him go when he got too expensive. Of course, they haven't won a SB since then...
I've seen some analysis saying that teams should concentrate more on getting guys who can kick-off into the endzone and get touchbacks than on guys who can hit long field goals. Everyone can hit short and medium field goals. A long field goal determines a couple of games a year, maybe, but kick-offs determine field position several times in every single game!