You can't take a bus directly from Downtown Disney to the parks! You have to take one to a resort and then go from the resort to the park.Doing that could take up to 2 hours,so you might save some money,but you're losing a lot of time in the parks!!
I've done this maybe 2 times and for someone who values their time in the park,it's just not worth it.Last time I did it I was only doing 1/2 a day at Epcot.We started the day at Sea World then went to Downtown Disney to eat at Earl of Sandwich,then took a bus to the Beach Club which took forever to come, then it had to stop at the other Downtown Disney stop then the Dolphin resort, then the Swan,it took at least an hour and a half.And to come back to Downtown Disney, I think it took longer,it was EMH at the MK and 6 buses came for Mk before one came for Downtown Disney at the Boardwalk bus stop.We wanted to eat at one of the Downtown Disney restaurants but it was so late we skipped it.We have Seasonal passes and come often so we did it since we wanted to eat at Downtown Disney and it was doing something different while saving $10,but it's honestly not worth it!!