You didn't specify year or engine, here's what's involved to change the plugs on my 2011 Avalon Limited.

How to change spark plugs on V6 Toyota and Lexus 2GR-FE 3.5L engine

 

2010 and 2011

 

As far as you want until it begins to misfire.

 

Not to mention MPG loss, performance degradation, additional emissions, etc. It's always best to follow what your owner's manual says. If they are 4cyls, it is really easy - almost anyone can do it. There are videos on YouTube. V6s can be a bit more trouble, but still not terribly difficult, you can find videos for them too. If you aren't comfortable doing it yourself, than forget about the stealership - go to an independent shop.

 

They're Avalons, so they have the V6 and they will just be a pain to do regardless.

 

Reliability and a well running car is priceless. Don't cheap out! Vehicle ownership is NOT cheap, if you have not come to realize this.

Get a factory service manual or a Chilton/Haynes and follow the "cookbook" on how to change. Torque specs are provided too.

OR, trade in your Avalon for an I4 Toyota. Spark plug changes will be easy and affordable.

 

I'd do it for $150 - you supply six plugs. This job ain't that hard.
I'm in SE Wisconsin.