I did termite and pest control work for several years back during summers in college. I can assure you that there is nothing available to the general public that will eliminate a cockroach infestation. I've even been to homes where it took 2-3 treatments of licensed chemical to get it under control. Once you have them bad, it's next to impossible to get it under control.
So don't waste your money on anything you can buy without a permit/license. If that stuff worked, you wouldn't still be having problems after all these years. Call a licensed exterminator and make sure they are using the required dosage/dilution. If you have them bad, don't expect one trip out to fix the problem though. And prices vary. It has been since the early 90's that I did it and back then, we charge $75 for the first visit.
Edit: There are some NASTY people out there. I remember going to homes where when I opened the front door, roaches literally fell off the top of the door and on to my head. If you opened a kitchen cabinet, you may see 20 of them on the inside of the cabinet door. Of course, these were the homes where there were rotten potatoes, onions and other crap under the kitchen sink and food debris all over the counters but yet they couldn't figure out why they had roaches. :roll: Not suggesting that's how you live, OP. Your post just made me think back to some of the homes I treated that were really bad.