Driving the price of an item based on # available

Driving the price of an item based on # available

I'm currently working on a web based game that has a marketplace in it;
i'm starting to add some functions so eventually users can buy and sell
their own items. The function i'm working on right now is how to drive the
price of an item up or down depending on the availability of said item.
the database has a "price" "available" and "number" column. (the price
column may be removed depending on how i go about this)..
anyway, I'm wondering if you guys have any input on how to go about doing
this? I haven't started on the code yet, because i'm not 100% sure how to
even go about doing it. i'm guessing just a doing a check to see how many
items are left and then updating the price based on that, but i feel like
that might be a bit too... barbaric.