In the US maybe around $60-70. You'll also need the proper tools, which include a Phillips #00 (#0 will work) screwdriver and a Torx T-6 screwdriver to remove the drive bracket. I know this says mid-2009, but the procedures are the same for every Unibody MacBook Pro up to the mid-2012 model.
https://manuals.info.apple.com/MANUALS/1000/MA1133/en_US/MBPRO_13inch_Mid2009_Hard_Drive_DIY.pdf
It might be worth it to get a USB3.0 enclosure if you need to clone the drive or if you wish to run Setup Assistant to transfer data. They less than $10 these days.
I personally had installed this model in my mid-2012:
https://www.bestbuy.com/site/sandisk-ultra-512gb-internal-sata-solid-state-drive/5969506.p?skuId=5969506
I upgraded to a 1 TB WD Blue recently because the prices were so cheap and I was running out of storage space. The performance is about the same. The WD and SanDisk SSDs are made by the same company, but the WD Blue has a 5 year warranty vs 3 for the SanDisk. On the Best Buy website you can use the menu to select a different size to see what the price is. This is the WD version:
https://www.bestbuy.com/site/wd-blue-500gb-internal-sata-solid-state-drive/6025802.p?skuId=6025802
There may also be issues if the SATA ribbon connector is worn. Nobody knows the exact reason, but SSDs seem to be more sensitive to worn connectors than hard drives (which can still have problems if it's bad enough). I've heard from some here that Apple proactively replaces them (free?) if brought in for something else. Other World Computing and Beetstech sell compatible ribbon connectors.
