I second Andy in suggesting the SILabs f12x and expand on the recommendation
1)My budget is between $100-$200.
the devboard cost $149 Do not let a few bucks "saved" on a devboard "scar you for life"
2)I want something that is easy to learn but flexible enough should my projects grow bigger.
easy to learn - a relative term, but the includede "ICE functionality" is worth a lot in learning. Be aware that the free Keil eval included does not allow >4k of code, you need to plan on a full compiler in the future
3)I mostly envision servo-control kind of applications.
what I suggest has all you need
4)on-board A/D converters.
the f12x has them
5)I guess 64k RAM and 64k ROM/FLASH should be enough for my apps.
64k RAM is enormous for a '51 application, the f12x has 8k which in 98.17% of the apps is plenty
6) some logic programmability in HW would be great to complement the microcontroller.
JTAG direct
7)what else I might need for control apps. ?
A PCA should be required the f12x has one.

In addition: the f12x is the fastest '51 derivative available, if your servo need ultrafast responde this may be the only game in town.

BTW STAY AWAY from the f3xx chips

Erik