Don't know how toned up and strong your arms are, but many cake mixes call for whipping eggs and sugar until light and fluffy, which can take up to 10 minutes using a mixer. That could be quite a workout by hand.
However, instead of investing in a stand mixer, which you say is not in your financial cards right now, take a look at some of the lesser expensive hand mixers. I have a 5-speed KitchenAid one that is a real trooper and it can be had at a reasonable price if you shop wisely. Especially if you surf Internet sites.
You might go that route until you can get a stand mixer.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!