Building a deck that fits your budget and is strong and fun to play is often a challenge, but with these tips and tricks, you can produce decks that won't empty your wallet, and will be fun to play.
Standard Format:
In standard format, you build a 60 card deck and face off against one or more opponents. Each player starts with 20 life and the goal is to deplete all your opponent's life totals to 0.
One thing that many players forget and don't use to their advantage when building a standard deck, is that you can have up to 4 copies of a card in a deck. This can be used to your advantage when buying cards and when playing your deck. Not only will you have higher chances of drawing into the cards you want, but it can help you when spending money on cards. Rather than get 4 different cards ranging 25 cents all the way up to $3, get 4 of the 25 cent card, and you will get more bang for your buck.
Next, is to not focus too much on your mana base. Many players will put expensive lands into their decks, and while this makes the deck stronger, it is much more expensive, basic lands and half-lands will do the job just fine.
When you spend less money on expensive lands, you will have more money to buy the cards that you want for your deck. For example, if you really want to put 4 copies of Champion of Lambholt in your deck, but each costs about $2, and you have only $25 to spend, but you have a $12 land in your deck, take out that expensive land, put in a basic or half-land, and you will have more money to purchase the copies of Champion of Lambholt.
Another thing to remember is that expensive cards often have cards that have similar effects but are less pricy, in those cases, go with those.
Finally, understand that sometimes a deck doesn't need a card. With standard, you have a lot of choices and freedom, but when you want a budget deck that is strong and can compete with your friends, leave out the expensive cards that may not really work with your deck, but are just there because they are cool.
Commander Format:
Commander is substantially different than standard, budget and play wise. More often than not, commander decks are more expensive to build than standard decks, because they contain 100 cards, and on top of that, you can only have one copy of each card, which means less easy to find those budget fill-ins. In commander, you start with 40 life, and you start with a 'commander' in the command zone. This commander can be any legendary creature, or a planeswalker, if the planeswalker says it can be your commander. I am not going to explain rules about commander, but I will link it below. Essentially, you have a 99 card library and 1 commander.
Alright, now for the tips. Let's start with some tips about deck structure. First, you have to select your commander. Many commanders are not expensive, and yet very strong. Keep this in mind when looking for a commander, and remember that in the end, you want a commander that is strong, someone you want to play, and fits your budget. When building commander, focus on building a deck with a sturdy mana base, but not an expensive one. Then, buy and add in cards similarly to how you would do it in standard, selecting cards that will help your deck and aren't too expensive.
After creating your base deck, you can play it a few times. I recommend you try it at least ten-twenty times before you make decisions going forward. You can make immediate decisions during those test drives, if you have the cards. After playing it a bunch, and getting to know it's strengths and weaknesses, you can improve it accordingly, buying cards that you need.
Finally, my last tip for deck structure is to do all that, and then buy some of the cards you want after playing the deck a lot. This allows you to save up for those cards and make decisions about whether you really need them.
Next, let's talk about finding the right card. I know it can be alluring to put expensive cards in your deck like Jeweled Lotus or Apex Devastator, etc., but sometimes (often), these cards aren't needed in your deck, and much more budget cards will do the job just as good, if not better.
Finally, many expensive cards will have substitutes that cost much less and do similar things. Below is a list of some substitutes for cards, you might find a card substitute perfect for your deck! (You can read the doc in the site or click the doc to open it in a new tab.)