hmm, 25 of these in parrellel
http://au.element14.com/international-rectifier/irlb8743pbf/mosfet-n-ch-30v-78a-to220/dp/1740785each taking 4A, would dissipate ~47W (11.6x4) means they would be running 74 degrees above ambient (silicon is 1.11c/w and assume 0.5 for heatsink), or 90-100 degrees
your load resistances would be 10 of these in parrellel per mosfet
http://www.jaycar.com.au/productView.asp?ID=RR1502&form=CAT2&SUBCATID=968#1 that will dissapate at half the resistors rating, 1.6W with a 0.4V drop
all mosfets must be insulated from each other by a sil-pad or similar, you will want a fan performing active cooling, but without selecting a capable heatsink, cannot calculate air flow rate,
the wiring between mosfets should be 2 heavy bus bars such as
http://australia.rs-online.com/web/p/busbars/4896568/this particular one can be cut to size, so you can cut it into 4 stips of 15 (13 mosfet legs, 1 tie in and a negative sense line for op amps) and have 2 groups of mosfets, 1 of 13, 1 of 12,
note i realise it would be quite difficult to solder to these, but insulated spade terminals generally fit onto the tabs and a piece of wire connected to them is very easy to solder to your mosfets and resistors,
so far we are up to only
$29.50 (mosfets)
$11.52 (resistors)
$34.70 (busbar)
$75.72 + postage
then comes op amps which isnt all that bad, it would just depends on how fancy you want to get with them,
the rest of your price will be the heatsink (jaycar do have one that forms and air column of sufficient size but isnt on there site, believe $40-70)
the case, the fan and current measurment,
an idea for current measurement would be to use an op amp (your already buy a crap load of them) and set one up as a 25 way adder, this will give you effectivly 0-10V (0-5 if in a voltage divider) for 0-100A, (yes your lowest and highest may get funky due to the op amp, but say you drove it at 7V and used the spec sheet to allow a 0V input / output,) to a standard volt meter
that is my thought on how to approach a 0-100A constant current load in a realistic size footprint,