I would hazard that most players wait for the techs to hit -5% from on time + neighbour bonus before purchasing a tech (at least the ones I watch do), and I feel like getting a tech for standard price a year early isn't exactly the most appealing. It could be fantastic a certain tech levels, where being the first into a military tech can give you a nice edge against the opponent, but otherwise I'd just wait that extra year and save the extra monarch points. Maybe make it 15%, so when you're a year early, it will cost -5%, same as if you had a neighbour with it, but being on time or having a neighbour with the tech level will drop 10% off your bonus, both being true will nullify your bonus entirely.
Might be a little complicated for a tooltip though