Firstly, you couldn't GIVE me a connection throttled to 512kb/s capped or uncapped. It would be like watching paint dry.
The maximum data you would be able to transfer in a month at 512kb/s is between 4 and 5 Gb and that would mean you would virtually have to be connected 24/7 with no hiccups at all. CellC offers the equivalent of that deal at the moment. For the equivalent of R375/month you get 5Gb/month. The catch is one has to pay R4499 for the year up front. For R599 you can get 3Gb valid for a year with the modem thrown in effectively giving you a modem for R224. I paid significantly less than that when the 60Gb/year deal was on promo but I will be more than happy to pay R4499 for 60Gb spread over a year when my present deal runs out.
If you want to pay monthly you can get 3Gb/month for R400 and that will bring you fairly close to 512kb/s uncapped.
Speaking for myself, if I was prepared to have my connection throttled to 512kb/s I would expect to achieve that speed 24/7 for both upload and download. Speed does, of course, depend on many factors but where I live I get over 7Mb/s download and 2Mb/s upload during off-peak times. Today's a public holiday and I have just measured 7.46Mbps up and 2.05Mbps down at 12:30. On a normal weekday I would expect a minimum of about between 2 - 3 Mbps down and about 1Mbps up at this time of day. Most often it is quite a bit higher than that.