I have a Mothra and I really like it. I bought it because it is one of the Vintage Tamas and because it is still good value for a vintage - not too much money.
I don't think Mothra's are particularly rare - they still come up pretty regularly on ebay at around the price you're looking at - sometimes lower - depends on whether you get a last minute run on bidding. Some of the Mothra shell designs are rare so they are much more expensive.
However, Mallow_Cup is selling one atm:
https://www.ebay.com/...9#ht_500wt_1131 and it's a good price and you know you can trust her...
Obviously the Osu/Mesu tamas need to be bought as a pair (boy/girl) if you want to breed them so they are more expensive and most people try to get the matching pair colours. (Make sure you buy a boy and a girl!)
That probably makes them more rare - or rather more in demand than Mothras - but of course, you can keep going with a Mothra and get more generations instead of them just dying and having to start over again (without having to buy a 2nd tama ...).
Some colours are more rare than others and that affects price, but if you just want another ancestor for your collection then rarity of shell design should not come before actually having one in your collection! XD
You probably already saw the other topic asking for advice on the Mothra vs Morino which might make useful reading to you:
https://www.tamatalk....__fromsearch__1 (there's the link just in case you missed it).