Well first of all, money doesn't buy love. If you spend more money on one, it doesn't mean you LOVE that child more. And you are right, older kids cost more. Their toys cost more, their clothes cost more.
I spend more money on my son than on my daughter. Why? Because my DD is *showered* with gifts by EVERYONE. She is disabled and we all know her time with us is short. She has 3 nurses that spoil her to death, buying her clothes and toys. So when it comes to her b-day or xmas, I don't buy much of anything for her, but go all-out for my DS since he doesn't get "special treatment".
If you obviously preferred spending more time with one child, or treated one child unkindly I would be able to see where she was coming from. But the money thing just doesn't even matter, especially with the large age gap.
Try not to worry about it. No parent tracks everything they spend to make sure everything is "equal". Most people provide what is needed based on what each kid needs!