Ive found that it pays to buy the best sac you can afford, and ive still got a Berghaus Dart 45L sac in my wardrobe, a bit discoloured, but should outlast me.
Its a bit discoloured, but was made out of bulletproof materials all those years ago, and before i moved up to North Wales several years ago, it was in regular use.
Nothing has failed on it over the years, even the zips are in fine fettle.
Granted it may have been manufactured close on thirty years ago, but the manufacturing quality was magnificent.
The choice of ruc sacs today, is probably just as extensive as it was back in the 1980s, but one does not buy a sac every few years, its a bit of kit that can last the owner a considerable time.
Just make sure you buy a sac of the correct size, with 50-55 Litres a great combination for a good allround day sac, or something that can be used for longer journies.
Within reason, set your budget to purchase the best sac you can sensibly afford, you will certainly not regret buying quality decades later.