AITA for not buying my girlfriends kids cars

My daughter is turning 16 and I agreed to help get her a car. She asked for a little honda or toyota thats good on gas. We set a budget of 3k and if she wants something more expensive then she needs to kick in the remainder. My girlfriends 2 kids got upset because I wont buy them the cars they want. 1 asked me for bmw i8 and the other requested a brand new truck. I told them they needed to take it up with their mom and dad that they are the ones that should be buying them a car. We aren't married and only been together 2.5 years. Their dad is pissed because i won't help out his kids and girlfriends pissed because I'm doing for my kids but not hers. Girlfriends sister and husband agrees with them saying I'm an asshole for treating her kids different. I don't think I'm the asshole but instead feel like I got a gold digger family after what I work hard for. Figured I'll let the internet determine if I'm actually the asshole and if i should reconsider my stance on the subject.