I've not heard CX180 or the PL21 and hence cannot comment about them. As for Philips, it is generally said that their costlier offerings do not improve on their cheaper offerings by much, though I do not have personal experience with them.
There's no rule in audio world that costlier is better. Brand value, marketing expenses, R&D expenses, support costs, even packaging costs all get factored into your price in the case of well-known brands. Secondly, the amount you spend is irrelevant when it comes to build quality and sound quality improvements. A $250 DDM2 failed quickly, a $150 RE262 needed a couple of RMAs within months for some, Monster IEMs despite their higher cost have poor build quality. As for SQ, in general, costlier gets you slightly better SQ. But, it is not always true. A $7 Monoprice 8320 sounds better than many $15 and even $30 sets.
Secondly, the term sound quality itself is left to individual perception. You can like what others dislike and vice versa - much like any other hobby.
All that said, you have two options. First is to go for ES18 or a SHE3570. In that case, if you find them unsuitable for your tastes, you might as well upgrade to something better in the future. But, I am saying that purely from understanding your sound signature preferences. Two IEMs, even spaced apart by an year, is two spends and hence amounts to something costly than a single IEM purchase.
As a second option, go for E10 / PL21. E10 is definitely better than either and most likely better than CX180 as well. Build quality of E10 is better than ES18. The only issue is if you don't find them to your satisfaction, then you've already spent a lot more than you originally intended and still left not as satisfied as you wanted.
