Jos There are no costs, apart from obviously needing a premium/joint/business/pack account with bunq to get access to the API (which isn't provided with free or travel card). I have been running this personally at no cost for about 6 months now...
IFTTT has a developer platform, which is free for your own use, and only costs money if you want to publish it to other users. But everything that I have shown can be done with the free version.
Google app engine allows you to run a number of projects (25 or something) for free, provided these do not exceed the free usage limits. Unless you have thousands of transactions a day, you should not come close to that.