The "proper" breadboard jumper leads that you've seen have metal pins soldered onto the end, so that they can withstand being unplugged and plugged in multiple times. Single core wire tends to bend as you plug it in once it's been used a number of times.
However, theres nothing wrong with using single core wire, and if it does start to bend you can just chop the end off and use the wire again.
The other benefit to the jumper leads is that they come in a number of different colours, which makes it easier to trace circuits, but again it's not a major issue if your wires aren't different colours, just makes life a bit easier.
I`ve bought a load of the jumper leads just for convenience, which were about $6 but that isn't likely to bankrupt you 