Hmm. I have just had a bass sent to me, via DHL and insured for 3000 Euro, and it costs under 70 Euros. I couldn't comment on the length of the parcel, but weight was 22KG. I was also offered another courier (don't know name im afraid) which cost 25 Euros but was only insured to 700 euros.
The seller either isn't looking hard enough or is trying it on it would seem.