If these are for quick flash type looking, you can get away with cheaper glass. If you are going to be spending more than 15 minutes at a time doing detailed surveillance, you need to put the price floor at $1000+. The glass under this price range will suck the eyes right out of your skull when you are spending lots of time behind them. Eye strain is real.
I gravitate hard towards Leica and Swarovski for this kind of work. Zeiss and Steiner come in tied for third place, followed distantly by others.
Light weight and glass quality are opposing interests. Generally, the heavier glass will be of higher quality, and offer a correspondingly better image, leading to reduced eye strain.
The larger objective lens will help with low light images, but you need to remember that exit pupil will come into play. In a mature adult with good eyes, this will average about 7mm. So, divide the objective lens dia. by 7mm and that will tell you about what the max usable power is for that given dia. So, a 50mm obj. dia./ 7mm exit pupil will max out the low light usability at around 7x. Ideal unit would be 7x50mm.
In that example, an 8x50mm set of bino's would be under kill (for low light) but not by much.
By this math, a set of 10x bino's would need a 70mm obj. dia. to be most useful in marginal light conditions.
I have been using Leica and Swarovski for more than a decade, in hard duty applications and have yet to mess them up. Butler Creek flip up lens covers help a lot. The 50-100 dollar Bushnells that I used before that went TU in short order. They were covered by warranty but you get crappy bino's replaced by crappy bino's in that scenario.
In my experience, the Euro brands are definitely the first choice.
