It was another colleagues 60th recently and no fuss was made of that!
That's what you mention, then. I agree with the PP who said how upsetting that must be for her, getting nothing whatsoever for her own landmark and then being asked for £30 for somebody else's. You say you know they won't make so much effort for your own big birthday. Aside from £30 being far too much in the first place, it absolutely has to be the same for everybody (unless they've specifically opted out or are nasty to everybody).
I wonder if the person organising it is playing the long game here. Does she have a big special birthday of her own coming up? And is she very good friends with the current birthday girl? Suddenly expecting £30 from everybody for her own birthday would be too obvious, but if she sets the benchmark for the birthday before hers, then benefits from the new 'going rate', and then, once she's had her bumper pay-out, tells everybody before the next one that she's had feedback that they can't afford/don't want to pay £30 each time, so it's now back to a fiver or a tenner. Of course, she wanted to give £30, to make it really special, but it's the others who are too mean to do it....
If she springs the old "But I've already bought the present for her on behalf of us all", don't let her guilt you into paying. That's entirely her problem for not checking everybody's budget beforehand - she doesn't get to dictate it.
Also, I'm not saying that she will, but it's far from unheard of in similar circumstances for the organiser to bank on other folk not adding up the total collected and to pocket some for themselves. Especially if you buy bespoke/engraved jewellery or something where the price isn't necessarily readily known. A present costing, say, £360 would obviously look like a substantial, expensive gift bought by pooling everybody's contributions, but would still leave a handy £150 'unaccounted for'.
For all you know, she might even have her own side business (or a family member has) selling what she's planning to buy, so that could be another motive.