Email us at supportzjb@scirra.com if you want to upgrade from Personal to Business and we can arrange a pay-the-difference upgrade. You only need a Business license once you have already made $5000 in revenue, so hopefully the additional cost is not a problem by then.
Considering Construct 2 gives you all possible platform exporters for a single price, it makes it considerably cheaper than buying the same set of platforms for GameMaker. The Master collection is $799, which is the equivalent product you get when you get either Construct 2 license.
GM is also quite a significantly different product to C2 anyway: C2's strength is in its much superior drag-and-drop event system, whereas GM has a focus on scripting. (You can also use Javascript in C2 with the Javascript SDK to make custom plugins and behaviors.)