If your plan already includes data, you cannot add more data to it as a top up and need to switch plans. Some of of the older plans flex up based on your usage but by the sounds of things you don't have one of those, provide more details on what you currently have and we can help you out more.
Your best option if you regularly change your data usage is to take their Shareplus plans as you can change how much take you pay for every month and they have options from 500MB to 10GB.
There is always a price per extra gig (overage), its just not as cheap as buying it ahead of time which is the behavior they want to promote.
The reason they don't sell unlimited data is due to the fact they're a public company and need to deliver continuous returns for shareholders via EBTIDA growth. Limiting your revenue (unlimited data at a specific price) while spiking your cost (customers using more data, growing over time) is a surefire way NOT to do this. SaskTel is not a public company but a Crown corporation and therefore has no such mandate.
There's a reason their parent company aren't reinvesting and its definitely not because they're too profitable:
Another potential reason they didn't invest in the spectrum auction is they can't justify the capital spend with their current customers' level of spending. There's a difference between someone offering something for a short time (even a couple years) and it being sustainable. Why do you think the US Telcos (AT&T and Verizon at least) no longer offer unlimited data?
I think we all understand that it is money driven, but as a consumer like myself that would like better rates, we do not care about share holders and there profit margin. We want better rates, I want unlimited data so I can switch all my internet to one provider instead of cable etc. I spoke to a telus agent and he said they are working on a unlimited package, see if that ever transpires.
I want an iPhone for $200 outright, doesn't mean Apple will ever sell it because they want 75% profit margins. I'd also like a lease on a Ferrari for <$500.
Think about your statement: I want unlimited data so I can switch all my internet to one provider instead of cable
Telus sells all of those things, why in the world would they want to give you unlimited data when this is what you plan to do after?
, we do not care about share holders and there profit margin. We want better rates,
You SHOULD care about shareholders, as were it not for them, the services you have come to expect would not exist. SOMEONE has to make the investment to allow the infrastructure to be built. Those folks take a risk in loaning their money to the company, and expect a return for their investment. Customers, on the other hand do not take similar risks with their cash, and see pretty much instant returns (services or goods) on their expenditure. Initial infrastructure cannot be built on sales income, as there is nothing to sell.
To add to what @NFtoBC said, you should also care about shareholders as you are most likely one. Telcos are a very common investment for anyone who has a pension. Something to think about...