It depends largely upon what kind of plants you are going to run. There are many plants that rely heavily upon nutrients in the substrate, like sword plants. Many stem plants will pull their nutrients directly from the water column, so having a rich substrate is not that important for them.
That being said, even if you are going to have some root feeding plants, it is still possible to run a plain gravel substrate and supplement it with root tabs and or laterite balls pushed under the roots.
Now, as for cheap. The cheapest and in my opinion the best plain gravel is what is called Silica Sand. It also comes under the name of Tex-Blast. The grains are available in plain sand, #1, #2 or #3 gravel. I prefer the gravel texture. The gravel is between 1 and 3 mm. I usually describe it as a very coarse sand or a very fine gravel. It is a joy to plant in, because the stems just stick. Plus, in a 10 gallon tank the grains are small enough to maintain the illusion that your tank is larger than it is. Did I say cheap? 100lbs for around $6. It can be found at many construction stores and is sometimes sold as pool filter media. You do have to be careful though because not all pool filter media is the same. Make sure it is silica sand as that is inert.
Pea gravel is available at most hardware stores, but in my opinion it is too large for planted tanks. That's not to say it doesn't work, but I always found it difficult for plants to get enough friction to stick. Fish or water movement usually uprooted them minutes after planting. It's also a little more expensive than tex-blast at $5 for a 40lbs bag. If you absolutely can't find tex-blast/silica sand in your area then I would go this route.