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.
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
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.)