Slughorn isn't bowled over by his salary even after his raise. When Harry takes Felix Felicis and runs into Slughorn before Aragog's burial, he explains what he's doing and Slughorn realises there's an opportunity to collect some venom and so goes down to Hagrid's, gets drunk and eventually hands over the memory. While he's talking himself into this we get:
'... seems an awful waste not to collect it ... might get a hundred Galleons a pint ... to be frank, my salary is not large ...'
Harry Potter and the Half-Blood Prince - p.450 - Bloomsbury - Chapter 22, After the Burial
However, the strong indication throughout the book is that he's doing alright for himself. He's accustomed to a pretty high standard of living and his idea of "not a very large salary" may differ a little from yours or mine.
Slughorn took another deep draught from his mug, his eyes moving carefully around the cabin now, looking, Harry knew, for more treasures that he might be able to covert into a plentiful supply of oak-matured mead, crystallised pineapple and velvet smoking jackets.
Harry Potter and the Half-Blood Prince - p.455 - Bloomsbury - Chapter 22, After the Burial
If you're asking me, I'd say probably enough to put you right in the middle of the middle class.
Real world teachers in Britain apparently earn £22,023 (new qualified) to £32,187. I would imagine Slughorn's salary is right at the top end of this range in the wizarding world (adjusted for PPP and all the rest of it) - probably slightly more.
Another thing to consider is that Hogwarts teachers apparently get full bed and board for all the time they're teaching. What this means is that if you don't have any dependants, a large percentage of your salary is disposable income. Which means that Slughorn's salary probably goes a lot further towards his cushy lifestyle than the salary of a teacher who was married and with kids. So the fact that Slughorn seems to have quite a nice life may be related to how much of his money is his to spend on sweets and comfy chairs. He, also, of course, gets quite a lot of gifts from ex-students.