I am happy to buy any number of DL miles at .2-.3cpm, and indeed I have bought millions of miles at that rate (through car rentals). $200+ for a "low" BE ticket to Europe is excellent value, and $400 for a almost-always available "medium" ticket would be bearable, too (although I've only had to resort to buying a partial "medium" ticket once in the past 30+ BE tickets I've bought).

DL occasionally has a 100% bonus offer on mileage transfers, which yields miles at about 1cpm -- $1000 for a "low" BE ticket to Europe is decent value, and $2000 for a "medium" ticket might still be considered bearable.

However, I would not entertain the idea of buying miles under the current 50% bonus offer. ~1.9cpm is waaaay too much for any airline's miles!