Let's take a look at the events at the end of FMA - I'm using the manga.
Hohenheim offers to use himself to get Alphonse back, out of paternal duty. Ed refuses because what happened to him and Alphonse was their own responsibility. Notice that in FMA, there's this sort of emphasis on personal responsibility for one's actions (e.g. Riza's comments about Ishbal, Ed's comments here, etc.). Consequently, it's possible to read Hohenheim's comments to Izumi Curtis as implying that he shouldn't - morally speaking - just give back to her everything she lost, given her own culpability. If I'm not mistaken, Ed has a similar attitude towards his automail leg at the end of the manga.
Ling also offers the philosopher's stone he has for the same purpose, but since Ed promised Al they wouldn't use it to get their bodies back, Ed refuses. This and Hohenheim's comments seem to imply that it is possible to recover what's lost from Truth if the appropriate toll is paid and what you're trying to recover actually exists.
Marcoh asks Mustang if he can use his philosopher's stone as payment in exchange for his sight, on the condition that the policies on Ishval are revised. It's not clear if Marcoh knows about Mustang having been forced to open the gate, but it is clear that Ishval is one of his motivations for putting forth this offer.
Given that it seems to be implied that Alphonse could have been retrieved with means other than Edward giving up alchemy, this seeming inconsistency is probably best interpreted as Hohenheim choosing not to completely restore Izumi Curtis in accordance with themes and ideas about personal responsibility in the series (since the state of Alphonse and Edward is also a result of their own actions).
While Hohenheim's offer to use himself to get Alphonse back may seem out of touch with his comments to Izumi, it's honestly not that inconsistent, particularly when he feels some responsibility for his sons' attempt at human transmutation and when this is an egregiously life-or-death scenario compared to Izumi Curtis'.
Moreover, just because there's some sort of emphasis on personal responsibility (and, at least judging from Riza's comments about Ishval, or those in the last episode of Brotherhood, the impossibility of atonement) in FMA doesn't mean that the characters don't try to improve on their own situation or that of others (e.g. Hohenheim's improvement of Izumi's blood flow, Ed and Al's pursuit of a means to get their bodies back, etc.). Rather, it seems more like a rejection of the attitude of "if I do X, I can make up for Y and forget about it". As thus, in the case of Roy, we should also note that Marcoh seems to have had fairly specific motives apart from restoring his sight in giving him the philosopher's stone.
