A "reasonable amount of gold" when compared to Harry's (but presumably less) plus everything inside Grimmauld Place.
Sirius left Harry his own personal fortune and possessions,
"But first of all I must tell you that Sirius’s will was discovered a week ago and that he left you everything he owned. (...) This is, in the main, fairly straightforward,” Dumbledore went on. “You add a reasonable amount of gold to your account at Gringotts, and you inherit all of Sirius’s personal possessions."
(Harry Potter and the Half-Blood Prince - Chapter 3)
The reasonable amount of gold probably came from his inheritance from his uncle Alphard, being that Sirius was imprisoned since he was 21.
Yeah, I camped out at your dad’s during the school holidays, and then when I was seventeen I got a place of my own, my Uncle Alphard had left me a decent bit of gold — he’s been wiped off here too, that’s probably why — anyway, after that I looked after myself. I was always welcome at Mr. and Mrs. Potter’s for Sunday lunch, though.”
(Harry Potter and the Order of the Phoenix - Chapter 6)
We do know that Sirius had purchased a motorbike and a firebolt broomstick, so the inheritance wasn't completely intact when he bequeathed it to Harry.
The only possession of note that Sirius had was Buckbeak.
There is also the matter of the hippogriff, Buckbeak. Hagrid has been looking after him since Sirius died, but Buckbeak is yours now, so if you would prefer to make different arrangements —
(Harry Potter and the Half-Blood Prince - Chapter 3)
Sirius also left Harry the Black Family house and slave.
"The slightly problematic part of the legacy (...) is that Sirius also left you number twelve, Grimmauld Place. (...) Black family tradition decreed that the house was handed down the direct line, to the next male with the name of ‘Black.’ Sirius was the very last of the line as his younger brother, Regulus, predeceased him and both were childless. (...) if you have indeed inherited the house, you have also inherited —” He flicked his wand for a fifth time, (...) “Kreacher.”
(Harry Potter and the Half-Blood Prince - Chapter 3)
Note that the house and everything in it was worth quite a lot.
“Sirius?” said Mundungus, who did not appear to have paid any
attention to this conversation, but had been minutely examining an
empty goblet. “This solid silver, mate?”
“Yes,” said Sirius, surveying it with distaste. “Finest fifteenth-century
goblin-wrought silver, embossed with the Black family crest.”
(Harry Potter and the Order of the Phoenix - Chapter 5)
He did not leave Harry the Black Family fortune as he never inherited it, having been written out of their inheritance. The house was an exception. Judging by the value of the house, if he would have inherited their fortune as well, his wealth would have easily dwarfed harry's.