In most cases you'll not only be saving money but contributing to recycling efforts at the same time. But after a few weeks they aren't so pretty anymore with all the buildup of minerals leaving a white crust on the inside surface. Try as you might you just can't seem to get into all the little crevices and ridges to get them clean. Here's a tip, let them soak in plain white vinegar for about 15 minutes. The acid in the vinegar dissolves the mineral deposits and with a quick wipe you can then just rinse them clean.
Same thing applies to propagating trays, watering cans etc.
By the way the buildup is a good visual indicator that it's time to give the plants themselves a good flushing with plain water. If the minerals are building up on the saucers it's probably building up in the potting mix as well. Flush the soilless mix with plenty of tepid water to redisolve and remove salts buildup. Your plants will love it.