  Assume that $(X,f)$ is a dynamical system and $\phi:X \to [-\infty, \infty)$
is a potential such that the $f$-invariant measure $\mu_\phi$ equivalent to
$\phi$-conformal measure is infinite, but that there is an inducing scheme $F =
f^\tau$ with a finite measure $\mu_{\bar\phi}$ and polynomial tails
$\mu_{\bar\phi}(\tau \geq n) = O(n^{-\beta})$, $\beta \in (0,1)$. We give
conditions under which the pressure of $f$ for a perturbed potential
$\phi+s\psi$ relates to the pressure of the induced system as $P(\phi+s\psi) =
(C P(\overline{\phi+s\psi}))^{1/\beta} (1+o(1))$, together with estimates for
the $o(1)$-error term. This extends results from Sarig to the setting of
infinite equilibrium states. We give several examples of such systems, thus
improving on the results of Lopes for the Pomeau-Manneville map with potential
$\phi_t = - t\log f'$, as well as on the results by Bruin & Todd on countably
piecewise linear unimodal Fibonacci maps. In addition, limit properties of the
family of measures $\mu_{\phi+s\psi}$ as $s\to 0$ are studied and statistical
properties (correlation coefficients and arcsine laws) under the limit measure
are derived.
