You're basically considering two different types of lenses: a "walkaround" zoom, and an ultrawide zoom.
The 18-55 is less expensive because it's also sold to go with a camera body in the box, and you can probably find a used copy for even less. It covers a relatively typical range for vacation shots: wide angle to slightly telephoto, so is likely to be more versatile for framing than an ultrawide, and more useful in a wider set of situations. But, unlike your pancake lens, its maximum aperture will be smaller than f/2.8 the more you zoom in. But its image quality is nicer than most other 18-55 kit lenses, which tend to be f/3.5-5.6 lenses, not f/2.8-4.
However, specifically for landscape and architecture shots, the ultrawide 10-24 is likely to be a better fit. It just won't be as versatile for other uses (say, portrait), since it cannot "zoom in" even as far as your 27/2.8 prime can, and on the whole, you still won't have any lens choices open to you that aren't wide angle, vs. the kit lens's 35-55 range being normal-to-telephoto for you. An ultrawide lens is also a bit trickier to learn how to use effectively, since certain decisions are more critical with a very wide lens, such as horizon placement in the frame (it determines apparent distortion), and foreground/background choices. Just using an ultrawide to cram more scene into the frame can initially have disappointing results until you learn how to use the lens.
In short, the 10-24 is a more specialized lens, and possibly a larger risk as a beginner, because you may not be sure if it's what you require, since you don't have experience with any focal lengths other than 27mm, and because it costs more.
The key factor is probably just to get experience with one or the other lens so you'll have a better basis for figuring out which one would be a better fit. Or, ask yourself when you're using the 27/2.8 if you want to zoom out all the time, or if you'd like to zoom both in and out, and are willing to give up some of the wider end for the cost savings and the ability to zoom in.
There are a few different ways you can proceed. You can save a bit more budget and consider renting the 10-24 before buying one (or asking to borrow a friend's) and try out the lens before committing to purchasing it. You can snag a used 18-55 and seeing how well it fits your purposes, and selling it if it doesn't work for you before moving onto the 10-24. Or you could start with an 18-55, and then save up more budget and eventually add a 10-24 to the bag if you find you'd like to have both.