My first binoculars for birding (we had some others, but these I carried around on all trips) cost $30 and I ID:d some 90 species with them. My current good pair were more expensive, and I am only a little further on my life list. If I divide the new birds into new binoculars, I get $6.66 per bird.
You can do your own math...
...an interesting valuation.
But what is the relative value of each bird... is each additional sighting "worth" more, perhaps???
Just another angle on the law of diminishing returns I suspect.
Mick