My dad hasn't even so much as bought me a card in years, never mind a present. He asks me to remind him when the family's birthdays are, which I do, including my twin db's who he buys for, yet he forgets me. Every year I take him xmas shopping for his dgc and he spends lots of money on them (£30+ each), but only about £5 on my dc. I remind him ds1 has his birthday a few days after xmas, but he won't buy him a present because he 'doesn't need any more so near to christmas'. It does seem unfair.
Every year, like you I spend time, effort and money on him for birthday, xmas, father's day etc. I used to get upset too, but over the years I've decided that that is the way he is and he's not going to change. I've gradually learnt to accept it, only rarely does it upset me now. Like you, its not the monetary value that bothers me, its the thought that I don't seem to care that much to him anymore. But then maybe in his eyes I'm all grown up now, and don't need to be cared for anymore. I guess I've learnt to stop judging him by my yardsticks, ie I show people I care eg, by remembering birthdays and buying gifts, just because he doesn't do it back, doesn't mean he doesn't care.
I know the logical solution, but haven't actually done it yet: spend less on him each year, and with the money I save, I should treat myself to something special in lieu of a gift from him. Sorry to waffle, but hth.