On my one Telkom SIM I get (checked these last week) :
Under LTE / LTE-A Bundle prices
10GB+10GB = R249
The 5GB+5GB=R301 is under "Once-off all networks" on mine, so you're better-off checking pricing once you get a SIM. You might be fortunate and get the 450GB offered at R59 (for 30 days) on the SmartBroadband Wireless option - just depends on the SIM you get ("luck of the draw", these days).
Once you've tested Telkom a bit, consider
https://www.rain.co.za/#/order - the off-peak (19 hours a day/night) is R249pm. There's a test period of 15 days if you get a SIM or 3, from them. I'm considering it myself. (in PE).
On Telkom, I'm currently getting anywhere from around 5Mbps (at its lowest) to a fairly consistent 30 - 45Mbps, but it does bounce around quite a bit on a big download. (this early am I got as much as 84.55Mbps, but sure that was for a split-second). I think your latency will be as much a factor to consider. Mine is just under 40 (ping), but it's slightly better than my ADSL which is almost 45.
Can you advise where you got your B315, and how much you paid?
(I don't think they're available in-stores these days, which is where I'd always prefer to purchase from).