Two banal observations:
1. Times one certainly buys border village: there's a card that costs less than 6 which you would prefer to gold if border village weren't on the board. Example: late game duchy, minion (often), wharf (often). Barring unusual circumstances (e.g. fear of emptying a pile), you'll always buy border village in these cases.
2. Times one certainly doesn't buy border village: there's no card on the board costing 5 or less such that you prefer (that card + village) to gold.
These are obvious.
The "trap" I'm referring to is for "in between" fives: cards you'd never consider taking over a gold, but might consider taking if they come with a free village. I've lost about a billion games within the span of the last day just by failing to compute which cards are worth picking up over gold (with a free village) and which aren't.
Any rules of thumb? My limited games so far suggest that in essentially all of these borderline cases, the free village isn't worth it, i.e., you shouldn't buy border village unless you're in case 1 above (buying something you'd already take over gold.)
Anyone want to argue a counterexample? This is a card that costs under 5, which you typically wouldn't buy over gold, but that you would buy over gold if it came with a free village.