Cost for a turnkey ERA slabside?

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The two price killers here are the "turn key" option that will cost you between 10/15k and the engine package that will run 5/7k. If you really want a milder 302 set up for cruising, there are several remanufactured engine suppliers that will give you a nice street 302 for 2k. Add a small 4bbl and mild cam and you are ready to run and will probably get 20/25 mpg as well as 300 HP. Same with a rebuilt Tremek trans, half the new price. Don't get too cute with the rear and brakes, as stock Impala brakes and a Jag Trac-Loc rear are more than enough for your planned use. You also don't need leather, as the vinyl seats are virtually undistinguishable from leather and wear better.
Use your head, get some quality guy to install the engine/trans combo (or do it yourself, it's really easy) and you can get a desireable slab ERA Cobra for well within your budget and Patrick's estimate. You can also spend a lot of cash if all you want to do is show up and drive away.
Sit down with Peter, he will give you all the options and recommendations you need to know. Congrats, you've made a wise choice going with ERA.