any cheaper alternative then dynamat?

Not really - any deadner material that works as well - or actually sticks to the vehicle in hot weather - will be priced comparably with Dynamat - figure a bit more than $100 for a bit more than a square meter. 

Don't bother with roofing materials either - like Peel & Seal or whatever - this stuff doesn't stick to the car, definately doesn't stick in hot weather - and it doesn't really work in the first place.

Deadening a car properly is one of the most critical things you can do for your stereo.  $150 for a case of Dynamat delivered is hardly pricey considering what it will do for your vehicle if properly installed - nevermind that Dynamat isn't surpassed in vibration reduction and exceeds all competitors in adhesion under extreme heat.  Do this right and don't cheap out!

EDIT:  Woot! 1k posts!

http://www.b-quiet.com/brownbread.html

But best bang for the buck, and something I've used successfully in my own car, comes from http://www.mcmaster.com/

Type 3427 into the search box at the top left.  On that page scroll down to sound and vibration damping sheets.  Choose mineral filled vinyl,  #9709T26.  Cost is currently at $24.85 for a 9 sq ft sheet.  Shipping is ground UPS and cost is WYSIWYG (several warehouses across the US).  If you prepare the metal the way you should do for any damping, it will stick for the long run.  I used a denatured alcohol cleaner and then preglued the metal surface with a quick swipe of 3M 77 spray glue.  Press and roll, press and roll.  Any wallpaper seam roller will do the rolling job for you just fine.

Oh, and this:  this company furnishes to businesses so start your own business when you fill out the application.  You'll need a name for it.

And here's a good study done on the subject of damping materials:  http://www.sounddeadenershowdown.com/

i've only read good things about this product, on another forum i visit. http://www.raamaudio.com/cgi-bin/index.cgi?p=pr

there's also these guys. http://www.secondskinaudio.com/

That company rocks.  They are a great place to find that odd item you can't find anywhere else.

At work we sell Hushmat.  It doesn't require heat to apply like Dynamat and sticks really well.  It is also a little cheaper. 

I've never used his product, but the owner of Second Skin always seemed to be a nice guy.  I doubt you could go wrong with that choice.

What about those temp rating stevdart?

I looked at that stuff when I was doing my car but the upper temp ratings on that page ~140degrees F - seems like it would be easily met/exceeded on a summer day in a parking lot. . .