Hoi An has a lot to offer local travellers on a budget, from stalls to restaurants.  

The central market food stalls are a great option if you want to try local fare all with fixed prices. Generally, a bowl of the local cau lao, mi quang (both are noodle dishes with pork and a little broth, but completely different styles), or a bowl of bun bo (beef noodle) should set you back no more than 25 000 dong.  You can also get an assortment of local dumplings (look for the stall right in the middle of the market selling local "Hue" specialities) for the same price.  Banh xeo should set you back around 10 000 dong/each, but they come with a huge plate of veggies and rice wraps so often 2 will be enough for a light meal.

Another favourite place for locals, and slowly foreigners, is the Ba Le Well Restaurant, down Ba Le Well alley.  For less than 100 000 dong/5 USD, you get all you can eat grilled pork skewers, banh xeo, local greens, spring rolls, rice wraps and a satay style dip: not to mention dessert (mango or chocolate pudding plus fruit).  It is a great place to go with a large group of people, which is obvious by all the locals there every night.

You can see banh mi vendors on every corner, generally selling a sandwich for 10-25 000 dong, but many vendors will try and sell for 20-40 to foreigners.  One exceptionally good banh mi restaurant which does not skimp on the fillings (and has its own bakery next door, so you're guaranteed to have fresh bread) is Banh Mi Phuong at the east end of Phan Chau Trinh road.  The restaurant's claim to fame is that Anthony Bourdain has visited and declared their banh mi the "best in Vietnam", which may very well be true.  If you're going to pay a little more for a banh mi, this is the right place to do it as you will go away satisfied.

If you have access to a kitchen, buying fresh ingredients at the local market should be an inexpensive experience, prices fluctuate depending on whats in season. The central market ladies are rather used to foreigners and will try to charge a little bit more, a good tip is to either use a more local market on the outskirts of town or to try to learn a few words in Vietnamese. The best time to shop is between 06:00 and 11:00 when everything is fresh. If you are shopping for fish, catches come in at dawn and then again around 16:00. Avoid market shopping between 12:00 and 14:00, most stalls shut up for a siesta and although vendors sleep at their stalls, they generally are not going to be too happy to give you a great price for waking them up. 

Sometimes people go on about 'foreigner prices' at street food stalls and market vendors. And good on them, these ladies are savvy market traders! But before you start feeling upset, it's worth bearing in mind that they do it to all new customers (especially local Vietnamese men). The more regularly you return to a particular vendor, the more likely they are to lower the price for you. 

The best tip if buying produce for the first time and you don't know any Vietnamese is to show them how much you have to spend and then point at what it is you want. This way you tend to get the local price as the vendor thinks you already know! No need for uncomfortable haggling and almost certainly insulting your market lady. And next time you know the approximate price of that item.