At the start of Harry Potter and the Half-Blood Prince there is a description of the place where Snape lives when he is not teaching at the school. The neighbourhood and the house itself are quite miserable.
Here some quotes:
Many miles away the chilly mist that had pressed against the Prime Minister's windows drifted over a dirty river that wound between overgrown, rubbish-strewn banks. An immense chimney, relic of a disused mill, reared up, shadowy and ominous.
The woman named Narcissa gained the top of the bank, where a line of old railings separated the river from a narrow, cobbled street. The other woman, Bella, followed at once. Side by side they stood looking across the road at the rows and rows of dilapidated brick houses, their windows dull and blind in the darkness.
"He lives here?" asked Bella in a voice of contempt.
"Here? In this Muggle dunghill? We must be the first of our kind ever to set foot--"
But Narcissa had rushed ahead. Rubbing her hand, her pursuer followed again, keeping her distance now, as they moved deeper into the deserted labyrinth of brick houses. At last, Narcissa hurried up a street named Spinner's End, over which the towering mill chimney seemed to hover like a giant admonitory finger. Her footsteps echoed on the cobbles as she passed boarded and broken windows, until she reached the very last house, where a dim light glimmered through the curtains in a downstairs room. She had knocked on the door before Bella, cursing under her breath, had caught up. Together they stood waiting, panting slightly, breathing in the smell of the dirty river that was carried to them on the night breeze.
Not a very appealing neighbourhood, right?
Now the description of the house itself:
They had stepped directly into a tiny sitting room, which had the feeling of a dark, padded cell. The walls were completely covered in books, most of them bound in old black or brown leather; a threadbare sofa, an old armchair, and a rickety table stood grouped together in a pool of dim light cast by a candle-filled lamp hung from the ceiling. The place had an air of neglect, as though it was not usually inhabited.
We know that Snape comes from a poor family and a professor's salary is usually not that big but most probably he could afford a much nicer place to live. If we compare to the Weasleys - they are also quite poor and have much more expenses with the seven kids. Nevertheless their home is a nice place to live without being a palace.
So didn't Snape buy/rent a decent home somewhere?