How can I include certifications that I have prepared and studied for but not yet earned
For the most part, I would say you can't. Or rather you shouldn't. While you can treat these as an upcoming graduation by listing an "Expected [Month, Year]" after them, it's not typical to do this. Most everyone accepts a claim that you'll be graduating at the end of the year. Few will think the same of a certification. The whole point of those is that you pass the requirements in order to call yourself certified.
When it comes to your resume, I would not list these as Certifications. Instead you can list the relevant skills in a Skills section. If the preparation is substantial or involves classroom trainings you can also list it in your Education section. That's especially true if these are well-known certification tracks that take a few months or years to complete.
But listing a certification you currently only expect to get would put a lot of people off.
But you absolutely can and should mention this in your cover letter! You can point to your preparation as proof of motivation to work in a certain field. Do avoid talking about financials as a reason you haven't taken the exam yet. Rightly or wrongly that could impact people's assessment of you. It likely will come up in an interview but you can be honest there and simply say you're saving up for the exam. Good and/or interested employers might tell you that they'll cover those costs if you were to start with them.