I hope you confirmed they are SAS2 backplanes in the 36-bay before buying, but as long as they are (same equipment batch, so probably,) that is a steal - free server with purchase of 2 backplanes!
Three years ago the only 36-bay unit I could find was packed by a fool in a giant box with a few phonebooks and packing peanuts and got really badly damaged of course. I got my money back after UPS took it away, but it took me almost a year to find another.
I jumped on the next 36-bay 847 units I found - they were bare with no PSUs or trays for about $200 each + shipping, which seemed great, but they had the SAS1 expander backplanes. Add $350 each for SAS2 expander backplanes, $40 each for a single PSU, and at the time $100 each for a set of trays was the best I could find, so ~$650 each after upgrades.
They served me well, and I really like how the motherboard tray slides out, so it's super-easy to trade boards around for testing and such, plus if you want power of 2 data drives in your VDEVs, 2 z2 VDEVs of 18 drives is perfect (36/2) - 2 = 2^4
Now I'm getting out of the 847 36-bay to use the 45-bay JBODs with an 836 3U 16-bay a head unit, that way I can use full height cards if I need to, and make 15-drive VDEVs, but I will miss the 36-bay units, they are especially well designed.