Below are a few very affordable portable digital piano's and keyboards.

Casio CDP-100

Eagletone EP100

M-Audio ProKeys Sono 88

Korg SP-170

Yamaha NP-30 (76 Keys)
Casio WK 7500 (76 Keys)

I own the little brother of the Casio WK 7500 and the piano sounds are very good on this one and also the keys are very good.