From an iOS perspective, and purely theoretically speaking (but trying to give you an idea of the questions I'd ask):
Before any sort of agreement I'd want to chat about exactly what you're setting out to do as the Apple approval process can sometime be tricky, and the exact parameters of your app can affect that approval (streaming versus video in the bundle (loaded in with the app), will the content change, is there anything about the content itself that could make approval difficult, etc). Depending on the specifics of what you want to launch, and whether the app registers a URL scheme, what you are setting out to do may or may not be possible. Apple secures each application by keeping it in it's own sandboxed environment which can make launching a second app tricky-- the second app would need to register a scheme with the OS which allows your app to call out to it.. The clock app doesn't register a url scheme that I know of, hopefully this isn't the app you had in mind ;)
Here is a reference of many of the common schemes:
http://wiki.akosma.com/IPhone_URL_SchemesNow if you wanted to play a video of your bank account emptying any time you tried to launch the custom SMS/MMS icon, before allowing someone to send a text, that would be doable!
Regarding cost, obviously it depends on just how involved you would want the developer to be vis-a-vis the approval process, how much if any design work would be necessary, etc. but if all you want is the code to be written that plays the video and launches the second app with no interaction, and for you to own it, I would probably quote a few hundred bucks ($2-300ish to write the app, ad-hoc distribute it to you for your approval or for personal use, etc.) with a probable turnaround time of 2-3 days. Realistically it would probably be a couple hours work in a single evening, but you'd want to build in a little "oh shit" cushion. If you'd want me to shepherd it through an Apple App store approval, that price would probably go up a few hundred bucks more (as a rejection, or multiple rejections can become time consuming).
Sorry if that got a little long, technical, or boring :)