A few of my thoughts -
I totally agree that this isn't about how much radishes cost to buy at the store - you could also easily point out that what I spend on my vegetable garden is more than what I would spend to buy vegetables at the store.
If your "room temperature" is too high, then no, radishes probably won't do well; our house stays around 68-70°F during the winter, which I think would be fine for the radishes. A little cooler would be optimal but they really aren't all that fussy.
Icicle radishes may not be the best variety to try growing in a milk jug; but you can give it a try and just harvest them before they're full grown if they seem to be getting too large. If you have a container deeper than a cut-off milk jug, I would use it - if not, go ahead and try the milk jug. Again, be sure you poke some drainage holes in the bottom (and then set the jug on something to catch any water that drains through). Use potting soil, not dirt from outside, and moisten it before filling the container. Gently firm it down to level the top, put your seeds on top, then cover with about half an inch of more of the moistened potting soil and pat that down. Lightly water the top - use a mister, or just carefully add a little more water over the whole thing - then cover with some plastic wrap (or the top of the milk jug or whatever) to keep things from drying out while the seeds are sprouting; if a seed starts to sprout, then is allowed to dry up, it will be dead and won't start growing again. Once there are some little green leaves popping up, take the covering off the container; if the plants are too crowded - they should be at least an inch apart - pull some of the seedlings out (you can rinse them off and toss them on a salad if you like). Keep them watered but not soggy. As they grow, if you left the plants about an inch apart, you can start pulling some out to give the others room to get larger.
Mostly, have fun! This is a good simple project to get a feel for how to grow plants from seed (and maybe even get some radishes to eat in the process).
