I have a project that requires me to trigger 88 pressure pads (normally open contacts) and to trigger a piano note for each one.
I'd like to use the Arduino(s) for this using MIDI.
Can anyone steer me in the right direction so I can at least get an idea of cost or the feasibility?
Many thanks
or the feasibility?
It's feasible, but it's not practical or economical to build a keyboard that a "real musician" would use. There is an Arduino MIDI library and you can find examples.
For reading 88 switches, look-up [u]multiplexing[/u]. For 88 inputs, you'll have to use multiple multiplexing-chips. If you want analog (for MIDI velocity) there are analog multiplexer chips.
I can at least get an idea of cost
If you just want MIDI (with the analog sound generated by a computer or other MIDI device) the biggest cost will probably be the "keyboard" (or whatever you are making with the pressure pads) and the cabinet. If you don't already have the parts, most-likely you'll spend more than the cost of a new MIDI keyboard. 
If you want analog audio output the costs could go up depending on the variety and realism of the sound. The Arduino doesn't have a DAC and it has limited memory & processing power, so any realistic sounds would have to be handed by additional electronics.