I usually go with giving my two girls ten presents each. This had been a tradition since I was a child. I like to find things on sale but what they want or need. I usually don't stress if I spend a little more on one than the other. But if I did find a bargain like half off on an expensive gadget or found it used on Amazon or EBay, I usually make a fair adjustment for the other child.
Many times the presents are usually equal in vale. But there have been times like when I got my older daughter a cell phone for Christmas and got her some smaller gifts to make the ten. Other times I got a gift the whole family could use, like a Play Station one year and a soda Stream another year.
Now that my girls are older with one finishing up her first semester in college and the other living on her own, I did things differently. I bought a laptop for my daughter before she started college and told her that that was her big Christmas present and took her shopping on Black Friday for clothes-so she knows what she is mostly getting for Christmas, but at least she got clothes that fit her and ones she likes. Plus got her a few smaller surprise presents.
My older daughter is getting mostly cash or a gift card that she can use for her apartment. Plus some smaller gifts.
Each year at my job I get gifts from work, plus purchase the company's products at the employee sale. I use these as gifts which I am giving some to my older daughter. I do base this on the retail price since they are new.