The volume of a sphere may be a logical way to determine equivalency, but it does not reflect the usefulness in the game, nor would it in real space.
A very large portion of the sphere is irrelevant because planets always end up in a roughly flat plain.
2,000 or 3,000 would be more appropriate, but what I'd really like is for it to also increase the scan range (and speed!) for identifying objects.
Given knowledge of how E

arranges systems, and of how (I like) to find distant suns, I'd probably make it something like:
Basic Scanner: 1,000 ls radius but otherwise exactly as it is now.
Intermediate: 10,000 ls radius (which is still not enough to detect a lot of secondary or tertiary stars, but should pick up nearly all local planets), scan time of object 1/3 faster, scan distance to object *3
Advanced: 100,000 ls radius (again, this wouldn't pick up ALL stars, but it would get most), scan time halved compared with basic, scan distance 5 * basic.
And then test and tweak until it feels right.
This would make exploring a bit more of an adventure, while helping to scale the speed/reward of exploring.
Exploring still needs other fixes of course:
Missions
Reward for identifying all objects in a system (10% bonus or something)
Increased reward for stars that are further from populated area
Increased reward for stars that have never been scanned before
Increased reward for
As it stands currently, the advanced scanner is sort of a handy-cap because it encourages you to go scanning distant stars and planets, which is slower than just hopping from system to system scanning stuff you can see from the nav point (usually within 500 ls!!)