First, I have no clue what something like this would cost to either fix or replace--you have to look to our contractor friends to give you that info. However, most owners require a security deposit before they rent out the unit. The whole point is to cover things like this. So I'll briefly address the issues having to do with your probable out-of-pocket to the LL.
It sounds as if the owner is a reasonable person, with whom you could have a reasonably calm, mature negotiation over this. The argument over normal wear and tear could rage on ad nauseum, not just on this board but in court, so it's better to look for a way to stay out of court. (Although bc has a really excellent point re: the absentee owner and the hassle of dealing with it through the courts.) At any rate, my point here (and I do have one) is that I wouldn't get into a big discussion with the owner over whether she should have anticipated this as normal wear and tear. Such arguments just tend to aggravate. (See Landlord's response to Jim as an example.) Instead, this situation calls for determining what we lawyers like to refer to as "nuisance value." That is, what will the owner take to avoid the nuisance of having to go through all the rigamarole she'd have to go through--and the associated costs--to sue you.
Given the age of the item, if you're correct about that, the arguability on the wear and tear issue, and the possibility of at least sprucing it up (i.e., Jim's suggestion on the stuff in the disgustingly bright can), it seems that the 25%-of-replacement-value suggestions you've been getting are a pretty good call on a place to at least begin. Going up to 50% is not an unreasonable expectation, assuming she's not replacing plastic with something superior and expecting you to bear the cost.
On the other hand, if you made a security deposit, you should probably expect the full value of replacement to be taken out of it. It's the easiest thing for her to do, you see.