If you read bin review sites and start looking at specialists, few better outlets stock anything much under £100. Budget would be defined as < £150, mid market £150- £350, and 'premium' £1000+. How the other half lives !
In terms of optical and overall quality, if one trusts the ratings given on a review site I found, it seems to buy a nice usable good budget pair with a 70% rating your looking at £100, to go to 'excellent' 80% will be around the £300 mark, and if you want that extra elusive few degrees of stunning clarity your restricted to makes like Swarovski which few of us can afford. The highest rated 97% Swarovski will set you back about £1500, or slightly less than I paid for my last 2nd hand Audi A4 car.
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I'm trying to justify spending £165 for 80% optical quality, I shan't be going down the polo club with them. I like the way the Vortex Diamondbacks will focus down to 4.5 feet, nice for watching the bees and flutterbies.
For anyone interested, this is the review site I've been perusing;
http://www.bestbinocularsreviews.com/