Another possible variation might be the quality of soil used. If you want to plant immediately, we don't have enough compost in the compost heap in the greenhouses (because they need it for class projects and the kitchen gardens that provide food for the school), so you would probably have to order it. There would be gradations in quality, and that would give you at least three different levels of budget to choose from.

Some of the rarer plants IF you want to include them require special additives (dragon dung, vampire blood meal, etcetera). If you want them to thrive, that is. Of course, you COULD plant them, and add the additives later, with the NEXT budget cycle. And then hope that they survive until then. If they die, you could re-plant with something less expensive, but then you wouldn't need the additives after all.

You could try to scavenge the materials for the raised beds rather than order it. Perhaps there's some lumber in some of the castle attics? Of course, you don't know how the wood has been treated, and some it might rot. Or you could find stone and dig it up. That would add to the time you need to get everything into the ground, of course.

These are just a few of the concerns with adjusting the design or varying the budget, of course.