Out of curiosity, what part of the USA are you located?
I have been checking availability on-line at the Lowes website and - so far- only my local store in Illinois is actually showing a price of $5.50 for MEK
Other stores in other states, such as Florida, Wisconsin,Virginia, Ohio,
are showing a price for MEK Substitute- but no price listed for MEK while the local Illinois store I bought from shows a price for MEK and does not list an MEK Substitute.
Not every Illinois store has it............and sale prices also vary as some are charging $6.99 per gallon.
Many Illinois stores list only MEK Substitute.
You may want to check other stores in your area on-line for MEK prices.
The store you visited may yet discount the chemical for clearance.
It appears the store manager has a say in how much he wants to charge for items that are being dropped from stock.