I value it when developed early more than anything other than the chancellor and queen, so I suppose 7-8, but I recognise its value will decrease as pawns are exchanged, while the rook (the next best piece) will increase in value, so I expect in endgame the rook to be better.
The hawk is an interesting piece.
I do realize how it's important for hawks to get out early, but their points are slightly to high. BTW, I am arguing with the rook/chancellor. I refer to the useless space far away as "Nowhere" i.e. a space outside the board where queen only hits 1 piece. capitalized N. Q/R/C are the only ones who can effectively reach Nowhere, and it's there just for kicks. So, a rook can actually reach about 40 useful squares, and a chancellor can reach 48.
It's not often in infinite one goes Q(999999999999999999999999999999999999999999999999999999999999999,999999999999999999999999999999999999999999999999999999999) That space is "useless"
Perhaps the value of a piece is some function on the number of moves..?
HGMuller made a formula to estimate piece value based on how many squares it attacks. But it is only valid for pure-jumpers (like knight), and he also said for "short-range" jumpers. The formula is:
value (in centipawns) =33(N) + 0.6875(N)^2
N = number of squares attacked.
For example, these pieces are jumpers, and their value is estimated as follows:
knight: 8 squares, value = 308 (3 points)
guard: 8 squares, value - 308 (3 points)
hawk: 16 squares, value - 704 (7 points)
My hawks are jumping all over in my game v. Aiden. The only problem is it's jumps are slightly hard to control. That's why I traded one for his rook. 
The thing is, a hawk covers infinity squares less than a rook, yet it's worth more! What?
there are infinity files for it to travel along in the begining of the game... so, yeah.
Try to prove that a hawk is worth more than a rook.
There is only a centre at the beginning of the game in infinite chess (since there are no corners or edges). Definitely evaluating a piece based on the number of squares it is currently attacking is the way to go
Central? There is no center in infinity... it goes on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on... and you get the point.
And don't forget to base it on the pieces it is defending also.
There is always a centre in infinite chess. It's where the pawns and king are.
There is no edge, therefore there can be no center. The thing about infinite is you don't even have to control the center... you can flank without developing a single piece, or develop backwards... and bringing the queen out early is sometimes a good idea!
@CaptainTugwash is leaning towards 7/8 or even higher. I feel that it is only 5/6, but can still be a useful piece.