Late spring and early fall are the absolute sweet spots for Prague and Budapest. The summer crowds are gone, the weather is crisp enough for walking all day, and you can actually get a table at a cafe along the Vltava or the Danube without waiting an hour. You are looking at about $3500 for a solid ten-day itinerary covering flights, places to sleep, and daily expenses.
Spreading that cost out over time makes it easier to swallow, but you need a plan so you do not still owe for the trip long after you are back. Let us break down how to map out the dates, book the big items, and handle the payments.
Remember that the introductory window is a strict deadline, not free money. If you still have a balance left when those 18 months end, interest will kick in on what you owe.