What area of the country? Would you ever consider hardwood? It's cheaper than composite and looks 10 times better. Low maintaining factor. You might want to go alittle bigger, if that is possible, such as 14 x 12. You'd be surprised at how much more that would work for you as far as seating/tables.
If it were hardwood, 12 x 12 would run about $4,000, depending on factors such as height, location on property, etc.
PT would be about $3000. This is in the southeast

Since this is a DIY project it appears you are only trying to compare the price of decking boards. A quick stop by a Home Depot/Lowes would show that PT decking runs about $1/linear ft for a 6" wide board & Trex runs about $2.40/linear ft. Some other composites run about $2/linear ft.

For a 12X12 deck you need 288 linear ft of 6" wide decking boards so that would be 24 boards @ 12'long X 6" wide. For Trex the cost would be about $700, for PT about $300.

Just a footnote to my original post. That price is for a complete deck, framing and decking, railing and stairs. Cost for decking material only in hardwood would be about $700

Like decker173 says, you need turn around room when you plan the size of your deck. This consideration is very important. To visualize, trace the size you are considering
right where you are putting the proposed deck with cooking flour. Put your table and chairs and BBQ with-in that perimeter and you will see that 12 x 12 is justs not enough.
As far as cost goes, you have to ask yourself if you want something to last, maintainance free and look ahead as to what you future add on plans to this deck might encompass.
the porch guy