We derive a priori bounds for positive supersolutions of −∆ p u = ρ(x)f (u), where p > 1 and ∆ p is the p-Laplace operator, in a smooth bounded domain of R N with zero Dirichlet boundary conditions. We apply the results to nonlinear elliptic eigenvalue problem −∆ p u = λf (u), with Dirichlet boundary condition, where f is a nondecreasing continuous differentiable function on [0, ∞] such that f (0) > 0, f (t) 1 p−1 is superlinear at infinity, and give sharp upper and lower bounds for the extremal parameter λ * p . In particular, we consider the nonlinearities f (u) = e u and f (u) = (1 + u) m (m > p − 1 ) and give explicit estimates on λ * p . As a by-product of our results, we obtain a lower bound for the principal eigenvalue of the p-Laplacian that improves obtained results in the recent literature for some range of p and N .