What internet data usage can I expect when mining in pools? I plan to connect my ordinary PC to a pool such as NiceHash or CKPool's solo pools and join the 'lottery'. I can run a few hundred Mhs; I know it's low, but that's why it's called lottery...

Before I commence my mining expedition, I would like to know what data usage I can expect, say by running it for 12 hours.

The research I have done on the topic only points to the fact that it is "not much" but given that data costs in South Africa is among the highest in the world, "not much" has a different meaning to us.

The way I understand these solo pools work is that they pre-package the blocks, send it to me and I start hashing. Given the maximum size of 1MB per and for argument's sake they push a new block to me every 5 minutes, that would be ~150MB in 12 hours, which is more than reasonable.

Is there something wrong with my reasoning? If we start to look at 500MB and above per day, that would start to infringe on our personal usage of the data.
No, it's much less than you think. If you look at the Stratum protocol, you'll see that the pool server doesn't send you the full list of transactions of a block, but just their hash (in a merkle-tree fashion). So, each new block that they'll send you is only a few hundred bytes, every ten minutes or so.

Your miner will likely report shares, but this should be infrequent (once every few seconds at worst), and this is a few dozens of bytes.

All-in-all, you should be looking at less than 100kbyte / hour.

PS : you can join the lottery or... you can mine for science at the 3SUM pool (disclosure: I'm its author).