I'm not exactly sure what you're asking. Could you clarify your question with an example? Are you talking about weapons like
dagger of venom, (+1 dagger with 1x/day poison effect) with properties that don't match the standard +N pricing scheme? I don't know if there's an actual rules for that.
As a
guess (and ONLY a guess, I'm sure sbdy will correct me
), I'd probably just look at the total price of the item and find the closest match on the
table of enhancement prices. For example,
dagger of venom costs 8302gp; that close to the price of a +2 enhance, so I'd assume it's effectively a +2 item.
Same thing for properties like "shadowstrike" (MIC), which costs a flat 5000gp. For example, a
+1 shadowstrike longsword is priced at 7315gp. That's pretty close to 8000gp, so I'd call it a +2 for purposes of adding more enhancements. On the other hand, a
+4 shadowstrike longsword is priced at 37315gp. That's a lot closer to the +4 price than the +5 price, so I'd probably still consider it a +4 item.