cost me about 50 bucks to throw together a nice stand with ply wood for 2 40 gallons. for a 75 your looking to spend about 75 bucks. For someone to put it together for you will prob charge u more for labor (unless they're nice) I say find an entertainment center on craigslist to suit your needs ^^ most things are built in standard sizes (i find i can fit a 55/75 on most entertainment units that were wood based)
forgot to add cost of paint to make it black! xD i went over kill with the stand tho.