JS has all the built-in math functions you need.
Here's a naive implementation of what you're talking about: MktoForms2 :: Client Calc
Ignoring the confusing field names (I do this so I don't have to clutter my prod instance with unused fields) the approach is that you have the rate stored in a hidden field (like you said) and then you prompt the user for the lot size. Only when they submit the lot size, you actually multiply the lot size by the hidden rate before submitting the form to the Marketo db.
You'd probably find it more visually appealing to have a second hidden field that stores the lot size * rate. Then you don't have to "magically" update the lot size field while the user is watching. Or you could just hide the form while you're submitting, same difference so they don't see that. Anyway, this is just as a quick-and-dirty demo of how you can massage field values any way you want before posting to Marketo.
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