the sales rep kind of concluded that the more you pay the better ventilation it will be which may well be true.
You're either misquoting him, or his possibly well intentioned advice was poorly worded. As with most spending to gain additional improvements of certain traits, such as ventilation, you quickly reach a point of diminishing returns when spending more quickly leads to marginal improvements. You don't need high tech (high priced) helmets to get sufficient venting, just one with forward facing vents, the bigger the better, but there are limits to what can be achieved and still keep the helmet functional
With winter approaching however, you might find this is a disadvantage, if it gets really cold, you might find large vents cool you down too much, unless you wear a warm cap already, in which case, consider riding without the cap. As you're not willingness to commit major funds to an upgrade, I assume you could do with saving money, so I were you I would stick with what you have if it fits well.
As other's have said, you'll not find a helmet that combats sweating, they all have those pads that absorb sweat, and the contact between the scalp and helmet doesn't let the moisture escape those areas, even with ventilation. You'll still be soaked in sweat. If you have a closed helmet now, simply swapping to a vented model on the cheap will bring a massive improvement in ventilation, but not stop sweating. I have a kask mojito RRP £125 IIRC, and still sweat profusely.
There's no reason to spend more than your lower end budget, unless it's for reasons of vanity.