I'd say two S3s are worth more than a Dragon miner even without the PSU.
I feel the trade is adequate as current prices for Dragon Miners are approx £400 whereas you can buy an S3 with PSU for between £150-£200
I am looking for 2 x S3's with only 1 x PSU as I currently have 1 x S3 running one half of the PSU.
I will sell my 1TH dragon miner right now for £400 but I think that's unlikely.
Keep in mind there is also a 1.5TH model.