I got a second hand Deore SLM590 right shifter on
ebay for £12 BIN price last night. Others I've seen have been in pairs, around the £20-25 second hand. But as I only needed the RH side it seemed a waste to buy a set. New sets seem to go for around £40-£60.
I did see an Alivio for £10 but thought I'd upgrade for the extra £2
New Alivio shifter sets seem to go for around £15-20 second hand, and new around £30 per pair set.
These prices are what I've seen browsing ebay for the last few days.