This is a fun question because there is a lot of choices involved. To make answering easy, I have included the following:
I thought to shortlist meals and dinners by using a requirement of defining a cheap meal as one medium-sized course and one accompanying drink costing either 20 dollars shared by two people or ten dollars for one person. This gets one focused on cheap meals and some really cheap meal choices to be described in the shortlist below. A great thing about this question is it's a chance to chime in with a new idea or meal idea. While my requirement helps me write a long answer, any reader short answers don't have to use my requirement. It should also be noted there are lunch specials galore in this situation - there are plenty of left field or unorthodox answers, hence a great reason to answer above.
Cheap Meals Shortlisted
The Breadhound , similar to St Lucia Bakery's situation but on Hawken Drive, is delicious yet affordable. With their social media offering linked above, they are quite established next door to IGA, again, similar to St Lucia Bakery, in that they've been in St Lucia for a while.
Dominos - at Mitre Street - have a great yet compact franchise in St Lucia, but surely it's more than acceptable to use as an answer, as they do a timely and efficient job there - on the topic of value fast food, there is also Subway (on Hawken Drive).
IGA - there are options there for ten dollars, especially if you work at UQ or are in a club there. Accessed by a walk on Picardy Street and either eat cold or reheat back at work. Yes, they are a fair addition as they meet the requirement for a ten dollar per person meal.
Indian Feast - definitely worth a mention. I recall their ten dollar deals and given that they seem to aim to cater mainly for people who want a cheap meal, probably still have that option available, so I'd have to shortlist them, at least for having that option in the last few years. Another reason is when as a customer there several times, I've always had good yet tasty meals with them.
Kaikai Chicken - had a good review response on Google, quite a lot of reviews as well, and I surely have to get this restaurant/takeout on the shortlist, but was it cheap? For the quality, it is good value, but it's still just ever so slightly beyond my requirement, with a lunch for one about $15 - unless they offer an impromptu lunch special, such as a cheap katsu and lemon tea.
Kurry Kingdom - is just above my requirement like Kaikai Chicken. Unless you order the butter chicken pizza and share a soft drink, the bill for two isn't under my $20 promise.
And, next door to The Breadhound is a super little sushi bar, called Misushi for sushi and a drink or a karaage. I really like that shop because it's compact yet they fill you up for cheap. Mr Deno Kebabs - it's a little over ten dollars for a kebab but share a snack box ($14) and two cans of drink ($3 each) and you have a meal for two people for 20 dollars. They even were featured in Brisbane's best kebabs from theurbanlist.com.
Misushi, St Lucia, source: yelp.com..
St Lucia Bakery - for lunch, yes, a pie and drink hits the spot for less than $10 amongst several $10 options for lunch.
St Lucia Supermarket - similar to IGA, it's possible, they have sandwiches and pies as well as packaged food for reheating.
St Lucia Meals on Wheels - while they are inexpensive, their food is not really readily available unless you genuinely need their help, but still worth a mention.
Zambeekas - I thought you could get something at this grilled chicken diner for 10-20 dollars per person with a grilled chicken feast for two for $24.85, or a chicken burger or chicken wrap and 375ml drink for $11. Zambeekas was just a hint short of my $10 per person requirement yet worth some promotion. The feast for two has a side dish but no drink. The drinks are $3 each.
There are other restaurants of note as well as a few cafés on Hawken Drive - if a few skewers and a soy milk is enough to get by, then Beijing BBQ house is under the ten dollar mark. There is a Thai diner on Mitre Street, at least one café (Briki) open for lunch near the Ironside school but is affordable for a coffee or cake, but I don't think they have ten dollar lunches. There are a plethora of options at UQ, including some of the clubs that have a sausage and bread and drink for easily less than ten dollars. There are also street food vans, and, UQ's soccer, swimming, tennis and rugby clubs can fill you up with some basic food and drink for less than ten dollars.
There are dining options at Gailey Fiveways and St Lucia Golf Links that aren't cheap for price and are not as near location-wise - although Crust, Nightowl, Tipplers and King of Cakes are all useful in terms of a ten dollar food and drink combined. Coles Express is a bit far as well, but with their pies, wraps, muffins, milk drinks and deals on Gailey Road will get one a meal for less than ten dollars. The food courts at the St Lucia Campus have lots of options and some of the bars on campus have been known to offer cheap food and really cheap beer. St Lucia Bowls Club was more for functions, I couldn't really see it as a cheap lunch option, yet such a grand old club, worth a hint of a mention.
As you can see, it's a fun yet thorough question for a writer to analyse and what appears to be a bit of a choice for a reader to answer, namely where is the best cheap lunch near UQ?
Note that St Lucia photo courtesy of the author, The Breadhound pie picture sourced from Facebook.com with KaiKai Chicken sourced from Yelp.com and Misushi picture sourced from Yelp.com