wrote:
My water heater is leaking around the base, and I can see that it's
rusted around the edge. It's 15 years old.

I'm looking around at prices, but does anyone know about how much I
can expect the plumber/installation to cost?

The cost varies widely according to the cost of labor in your area, code
and permit requirements, and whatever additional work may be needed.

And my house is for sale, so now I have a puddle on the basement floor
if we have a showing before it gets replaced.

On the other hand, I get to add "New water heater" to the listing.

A bright light at the end of the tunnel. If you're using a real estate
agent, s/he may offer some guidance.