Without even checking what they're worth new, I'd suggest around 2/3 of new price. Any more than that and people will be concerned that they're missing out on the warranty.
I'd say half.
I've sold a good number of welders which are usually around the 4-5 grand mark new (pedals, coolers included) and I've always got 50-60% of it's new price tag back as a no warranty offered sale.
It all depends on what someone is willing to pay and whether there is someone on the lookout at the time you are selling. Not very helpful I know but as an example I put my MTS255S up for sale with the spool gun, price new incl the spool gun is £1237 and I priced it at £800, so 2/3 of new price. Mine is only 7 months old so still well over a years warranty and I got no interest.
Not too bothered really as I will just hang on to it as a spare but it just shows that even the 2/3 rule needs someone wanting one at the time you are selling.