Great...but could have been better.
Great sound, good sprite work, and a genuinely funny storyline...you have all three, and I must say I am impressed.
There is room for improvement, however. From the way it's written, it makes me believe English is your second language. The whole dialogue seems to be a bit "off." The characters speak to each other in words that have a formal connotation. As a result, while the story is funny, the dialogue is a bit lifeless.
Second, while some of the visuals look great, there were moments where I believe you could have done better in your choices. For instance, when Mario gave Yoshi the money, why not use a gold coiln, and rewrite the dialogue as to not include "real world" monetary values. Also, when Yoshi climbed the stairs, his movements were a little off. There are other things that bothered me, but I hope you can understand what I mean.
Don't get me wrong, this Flash is very good. It's just these little details prevent me from giving it a five.