TextCell[" "] is the problem.
f[a_, b_] :=
10 Sqrt[5] \[Sqrt]((215. + 43.566 a +
37.336 b) (0.25 (5/(2 a) +
1500 (1/800 - (-1 + a)/(1000 a)) ) +
0.25 (5/(2 b) + 1250. (1/700 - (-1 + b)/(1000 b)) )))
So f[1,1] yields: 565.986
You rule assignment would work for the expression, e.g.
f[u, v] /. {u -> 1, v -> 1}