I only have one child so perhaps I don't really know what I'm talking about - but it doesn't seem essential to me to spend the same amount on each. The shirt is I assume a rare item it was hard to get hold of - a sort of special, once in a lifetime gift this guy will be absolutely thrilled with (at least one hopes so).
It's not realistic to find something so special this year for each of them, and also not necessary I don't think? Another year you might happen upon something perfect for one of the others, and so on....
I do have a brother and I have never even considered for a second how much mom and dad spend on us, and whether it's equivalent. In fact I strongly suspect it isn't. One year they got my brother a laptop because he needed one, that would have cost way more than the usual clothes/books mom gets me.
This year she is getting me a phone, because everyone in the family is frustrated by my shitty phone with it's crap camera that takes awful pictures. I don't even especially want the phone, but DD, DM, probably even DB, strongly want me to have one, and it IS very generous of them, so....
My point is, some years I need something special so they spend more on me, sometimes it's the other way round, other years we both just get more everyday gifts - we just don't compare, especially in terms of cost.
Plus, will your kids and their partners even know how much the shirt cost?
Why don't you just buy them all something you think they will really like, without worrying about the cost (as long as you stay within your overall budget of course) ? As long as nobody is just given a chocolate orange - everyone gets a decent, thoughtful gift - it's not about the money.