We have too many ERs to make a specific guide for them, but I agree that they tend not to be as date-based. Perhaps they need a range or something. It's hard to get a feel for what they are currently valued before because ER is a new rarity.
Edit: I went through the archives and couldn't find any ERs newer than 2012. So ER needs a combination of a set price guide plus a list of specific pets.
So this is Draft v2 with both your feedback incorporated:

I'm not confident on the VRs still. This new chart removes overlap between rarities though. It's how I thought it should work originally, but a lot of people were saying they thought older pets should fetch higher rarity newer pets, so I'm not sure if this is right either.
I couldn't find any good guideline for ER.
I did find some valuations:
2012 ERs
July Lioness 750
August Lion PPS 1000
2010 ERs
May Lioness 600
But nothing up to date for pets like these, so I think 2011, 2010 and 2009 (who have ERs outside the lions) need a set valuation for ER to cover these sorts of pets.
2011 ERs
New Year Dog
Vday PPS Horse