honestly i'd suggest spending a bit more on your rack as you want something rather sturdy. the $20-40 racks are built very cheaply.
you could probably get away with one if you used the flat wall mount style but if you want articulating or an arm style then you definitely want to expand your budget by quite a bit to get something decent.
i paid retail price for mine... but i got such a good deal on the tv that i couldnt complain. i bought a $2000 tv for $1900 but with a $160 wall mount and an extended 5 year service plan. i probably could have paid half online.. oh well. it is one of the hammered finish flat wall mounts that is very heavily built (i could hang off it) and i wouldnt use anything but for my own televisions.