Okay, stop by your local PetCo. There 2.5 gallon tank (lighted hood, filter in the hood bow front betta tank) is on sale for like 50% off. So depending on the price at your store it could be $18 or $25. But it's SO worth it.
If you can't afford that, or your parents think it's too much for a single betta, get the walmart Hawkeye 5 gallon for $25 then you can put a divider in for two bettas!
As for gravel, ditch GRAVEL. Ditch sand, too. Those are better for much larger tanks. Because they need basically a 100% weekly and a 25-50% sometime during the week, too, it's a PAIN. Trust me. Get larger pebbles or go for "river rock". The larger pebbles anchor REALLY well. I'll show you a picture of my male's tank with it and hopefully I have a picture of my female's with dollar store "stones" in it. The pebbles hold down his live plant extremely well and the stones in my female's hold down her fake plants wonderfully as well. Plus, I just dump it into a strainer and rinse it out with hot water really quick. The larger size of them means they don't stick, either, unlike gravel!
Plants. My first option? Go live! Anacharis is a super beginner plant and don't need anything fancy besides light. They are generally $2-$3 for a whole bunch of it. They also reproduce on their own! It can stay floating or can be tethered into gravel. It's perfect for decorating a smaller tank. Also, moss balls are decently priced and take next to nothing to maintain and flourish. If you're going fake, definitely go for silk plants. WalMart sells about 3 in a package for $5!
Thermometer is a must and are $2 at Walmart, too! Heaters go with this. Unless you live somewhere where it's reaaaal hot (like where I do!) you have to get one for your fish. Heaters vary from $12-$30, but for a smaller tank you can get one for about $12-$15. Look online for deals, too. You'd be surprised how much cheaper you can find them on online stores! Also, pet stores regularly have heaters on sale. So it pays to wait and look around.
I wouldn't use real beach items. They can affect the hardness, pH, etc. Not good for the fish! Plus, you don't know what's in them or what animal or species it came from. There are plenty of good, cheap things out there. I got a cave for 99 cents for my girl's tank from PetSmart! My boy's skull was on sale for $3 at Walmart. Just keep your eyes open! Plus, things like a terra cotta pot or a mug are cheap and can be used as a hidings spot.
Here's my male's tank broken down:
$5-Gravel (5lb bag, only used half!)
You can do this on a budget.
Not to mention, thrift stores and Craigslist are wonderful places for cheap or almost free tanks. I got a 1.5 gallon tank plus filter for $3 at Goodwill and there's no cracks or leaks and the filter works! On CL I've seen full set 10 gallons (heaters, lighted hoods, filters, etc.) for $20.
^ That's my boy's tank. It provides ample hiding space (with the skull) and the anacharis provides him somewhere to rest. I see him laying on it allll the time. Plus, because it's "bare" but extremely functionable, he actually gets to swim! It's not flooded with things. Half the time I can't find him in there, LOL! Those pebbles I found at PetCo, by the way.