So I was just curious about how much it would cost to rent a max level almost-complete deck.

So I went through peakmonsters.com's rental page and selected one of everything at max level.

This is what I got:

74 Level 10 commons 2141
71 Level 8 rares 2048
42 Level 6 epics 1904
51 level 4 golds 4110
Total 10,203 DEC/day

On average, that works out to about an 18% rental cost compared to buying.

That's... surprisingly not bad.

There were a few cards I left out because the prices were ridiculous and a few more that still had no offerings. But overall this covers beta, promo, reward, untamed, and dice editions. You could cut this down a lot by being more selective. Maybe you don't need every legendary summoner. Those are by far the most expensive.

Also, don't ignore the gold foils. There were a few instances where the gold foil card was cheaper than the regular foil. And plenty where the difference was pretty small.