While I might not know the exact answer to your question, I can tell you this:
I recently upgraded and got me an iPhone 4 on Vodacom (04/06/11)
It works out cheaper on the Talk 200s than the talk 75s. The lady at Vodacom was very helpful and we did a calculation.
The reason is simple, on the talk 75s you have to pay an amount for the package (let's say R75 <-- not the actual price), plus, let's say R249 per month for the phone on top of that (the lower the package the more you pay in on the phone) - So it comes to R325 per month.
On the talk 200s, you pay R200 for the package and only R99 per month for the phone, as the phone is now better subsidised by the higher service you signed up for. So in total you end up paying R299 per month, and you have an extra 125 minutes and an extra 110MB of data.
Please note these aren't actual figures, but an example of how they work.
What I'm trying to say here is: The high end phones are actually cheaper on the more expensive packages. You will probably get a better deal going for something like the Talk 200s than the TopUp 135.
I hope this info helps.