All helmets have to meet the relevant safety standards, so any helmet on sale will do. Once you get past that, you're left playing with things like weight, aerodynamics, ventilation and price. If yours not racing or TTing, aerodynamics are not all that crucial. The difference in weight between 200g and 300g isn't going to break your neck or make much difference to your climbing speed or whatever. Ventilation can be a concern, particularly in the summer or if you're riding somewhere warm. Price is price.
The way the trade off works is that the manufacturers have to comply with the regulations, so to create a helmet that is super light and super well ventilated costs more than a heavier, less well ventilated one. The safety is a fixed point, so you're not in the usual "3 criteria, pick 2" territory. One is already picked for you, so it's up to you whether you want something light or cheap.
Oh yeah, looks. How vain are you? Do you care how cool a plastic helmet looks? Assuming you're already wearing Lycra, sunglasses and funny shoes, there's limited ground to be made up here.
For what it's worth, I have a Giro Transfer helmet which cost £20odd from... a seller on eBay, but like a new boxed one, not just a random one. It looks alright, it's not massively heavy and it's fairly airy, though if there is a weak point at there. It does have nets in the vents which are useful for avoiding the catastrophic bee in helmet scenario that people occasionally recount.
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