Modify/add to price based on custom field? Is there a way to modify a variant's price based on a custom field in the product? 

To clarify, Let's say I have a matrix field with a block that has a text input with the handle name and a numeric input field with the handle price. This is then outputs as a radio button select list in the template like "Original" and "Name". When the "Name" radio button is selected, the price is picked up from the field price in that matrix block. The user could then add to cart with the modified price.

Any way to do this without plugins, or is there a plugin available for this logic? I'm not PHP savvy, so any light that can be shed on this is much appreciated. Ideally there's a way to do this within the template.
You could simply use variants to handle this couldn't you?  A non-name and named variant, each with a different price.

There's no other built in way to do what you want without PHP, I think.  The PHP would not be too complex though, if you fancy adventuring into Craft plugins, which are super-useful with Commerce in general, opening up all sorts of extra possibilities.