1, Which MCU is the cheapest in cost, and how much it cost.
depends on where you live and quantity, The Philips LPCs run about $1.30 to $0.52 in the US depending on qty.
2, Which is the easily available MCU. And how it can obtain.
depends on where you live
3, What do you think is the best MCU for above two limits i.e. cost and avalibility.
see above

Erik