Under $25 is a little hard... I can get with a few dollars of 25 though!
The RAT they mentioned, I've heard good things about it though never tried, is around $30:
http://www.amazon.com/Rat-Folder-Black-Blade-Plain/dp/B001E8EM2E/ref=pd_sbs_k_2
others:
$26:
http://www.amazon.com/Kershaw-Skyline-Textured-Handle-stone-washed/dp/B001CZBDF8/ref=pd_sim_sg_1
Under 25!
http://www.amazon.com/Spyderco-Lightweight-Point-Blade-Handle/dp/B00190TM68/ref=pd_sim_sg_6
27.77:
http://www.amazon.com/Victorinox-Sw...00VY4OJS/ref=pd_sim_sg_40/192-9262792-8521622
Hold on a second before buying, though. A few things to keep you safe and satisfied.
First, check the relative knife laws of where you're traveling. Many will have different and confusing laws. For instance, not only are you not allowed to have a blade over 3 inches in Britain- it cannot lock either. So, read your laws- or better, ask us about them.
Second: in terms of cutting rope and carrying around: cutting rope, if you're doing it a LOT will dull the knife rather quickly. By a lot I mean you're working as a sailor or something- using it 10 or more times a day. So, sharpening would come into play, as even the best knife probably won't still be sharp after a summer of intense, intense usage. Then, I'd recommend serrations if you're doing so, as they can be helpful and help with keeping an edge longer- though they're hard to sharpen.
If it's for occasional use- cutting an apple, occasional rope- you use it once a week or less- then I'd be comfortable telling you a plain edge is better. Actually, if you are going to cut rope a lot but still want to cut ordinary things, I'd recommend a combo edge- half serrated, half plain.
Let us know what you choose!
Zero