Found it pretty simple.
Yah, I hate when people say that too, but I guess I'm sorta proud that I worked out a way to solve them easily. Basically, first I'd put up the amount in the largest denominations possible (the fewest coins). Then I'd adjust it based on how many more coins I needed.
Switch half dollar for dimes = 4 more coins
switch quarter for 3 nickels and a dime = 3 more coins
switch quarter for 2 dimes and a nickel = 2 more coins
switch a dime for nickels, or a half dollar for quarters = 1 more coin
I got to level 12 before I quit... I kinda wish you had told us how many levels there are. It's very attractive. The manipulation is very satisfying, the realistic coins and background mesh very well with the colors and format you chose for the readouts, the sounds are pleasing. The time aspect was an excellent addition to this type of puzzle. Flawlessly done.