Multimode transceivers are still a slight bit cheaper ($30 for 10GBASE-LR vs $22 for 10GBASE-SR on fs.com) but that's more than offset by the single mode fiber being slightly cheaper, even for short distances.
SR is for when you accidentally installed MMF really.
Except, no, you can run (single lambda, not muxed) LR transceivers on MMF, it's out if spec but you actually get around +20% range before hitting dispersion limits.
I'll also note SR ranges are not in fact long enough for larger buildings. And this gets worse with higher speeds, since modal dispersion is a bandwidth problem, not attenuation.
Also, using 800G for comparison is a bit ludicrous ;). 10G is bread&butter, 25G/100G is mainline. (40G is the shunned weirdo uncle.)
Historically, the price difference between MMF and SMF transceivers was greatest at the highest speeds. That doesn't seem to be the case anymore for short-range transceivers.
For short runs inside a building, the transceiver prices are much more comparable. Compare https://www.fs.com/products/251819.html?now_cid=4089 to https://www.fs.com/products/282973.html?now_cid=4089