Recipes
As far as salsa recipes go, they're all over the web. Check sites like allrecipes.com or epicurious.com - - or justrightmenus.com!
Shelf Life
Refrigerator. With regard to shelf life, according to stilltasty.com, you're only going to get 5-7 days out of fresh salsa in the refrigerator.
Freezer. The same source says you can keep it for 1-2 months in the freezer. Use containers meant for storing frozen foods (they make plastic lidded containers for this very purpose), and leave "headspace" in the container (room for expansion as the liquid freezes). If you opt for freezing, you can use any recipe you'd like. Food safety won't be an issue so much as quality would. For example, I would not recommend freezing a melon salsa, though it's great fresh.
Canning. You also could pressure can your salsa. The National Center for Home Food Preservation is the best resource to learn to can. If you opt for canning, be sure to follow a trusted canning recipe - you can't just can any old recipe. I would use one off the National Center's site.
Cost Comparison
It all depends on what you make and the price of your ingredients, of course. If I can get cheap salsa at cost X and expensive, tasty salsa and cost 2X, my goal in making my own is to achieve great taste at a cost less than 2X. My homemade blueberry jam costs me about $2 per jar, much more expensive than generic jam; however, good blueberry jam is about $5 at the store.
Incidentally, I'll be making salsa this summer once I can get lots of cheap tomatoes. Keep an eye on jessica.mcrackan.com late in the summer if you'd like some real examples and cost comparisons.
A standard tomato implies that there is a standard salsa, which there isn't - but let's stick with your specific case of pico de gallo AKA salsa cruda.
The distinguishing characteristic of salsa cruda is that it uses raw tomatoes - the cruda literally means raw. Since you aren't going to be cooking them, and since water is going to be your primary binding agent, you'll want to use plump, juicy tomatoes that have potent flavour when raw, and that just so happens to be those bog-standard globe tomatoes you see in the supermarket aisles. Of course, if you can get garden-fresh tomatoes, so much the better.
Cherry tomatoes are also a great choice - they're very juicy and a little sweeter than globes - the only downside is that for a chopped salsa (as in pico de gallo), they tend to make preparation much more difficult and messy. If you've got the time and patience, give it a try; cherry tomato salsas taste much "fresher" than those made from globes.
The other common types of tomatoes, most notably roma and pear tomatoes, are really meant more for cooking. That's not to say you can't eat them raw, but they don't have a lot of juice or raw flavour, so they don't make a good base for cruda. Roasted roma or heirloom tomatoes (especially fire-roasted) make a great addition to salsa, but of course, it's not really "cruda" anymore if you cook any of the ingredients.
Do yourself a favour and don't use the plum tomatoes in a can; they're plenty juicy but have almost no seeds and no flavour.
So, all in all, for pico de gallo I believe that regular globe tomatoes are the most appropriate, but any tomatoes with strong flavour and plenty of juice will do.
Best Answer
Acid is your friend here. You have some lemon, and tomatoes are acidic, but apparently that isn't enough. You should get at least a good week out of fresh salsa (mine lasts longer than that). Try adding a good shot of plain, distilled vinegar. Many recipes for salsa (including my own) include vinegar; add as much as you can without negatively affecting the flavor.
A bit of salt wouldn't hurt either, again, just to the maximum for good flavor.