Need help with replacement saddle advice!

[Feastie]

So some unspeakable ******* came in the dark last night and stole my crappy second hand saddle off my crappy second hand bike, along with a load of bungee cables I bought at the weekend, ironically to replace the bungee cables I had nicked just last week! After 3 years of never having any of them stolen. Aaanyway, I digress....

I just want some advice about buying a replacement, and as I'm fairly penniless at the moment I just want anything functional which is cheap.
I've found this as the cheapest saddle £5 from Halfords, even though it's in white:
http://www.halfords.com/cycling/parts/saddles-seat-posts/san-marco-ponza-power-bike-saddle

And the thief seems to have taken the seat post too (unless I'm misunderstanding bike anatomy). This is the cheapest I can source, also Halfords:
http://www.halfords.com/cycling/parts/saddles-seat-posts/cinelli-pillar-seatpost

My questions are:
1) Does anybody know of any cheaper options for the seat post? I am very strapped for cash!
2) Are these the only two things I need to buy? When I look at pages on saddles & seat posts they're all selling seat clamps and screws of several varieties, and I don't want to end up with a seat post and a saddle and nothing to stick them together with.
3) When selecting the diameter of the seat post, do I measure my own bike from the two edges of the tube, or from the inside edges of the tube (i.e. the hollow bit only)?

This is a photo of my bike as it currently stands, saddle-less. I don't know if you can see, but there's a little clamp left on there, is that all I need and the saddle/seat post should slot together just fine?
http://i62.tinypic.com/2z7e8mr.jpg

Weirdly they also left this mysterious red wax where the seat post used to be. Still not 100% sure what that is. Anyway, advice appreciated, this commuter would like to be back on the road and not paying for buses they can't afford!!

 

[I like Skol]

Scum bags!

That is a reasonable price for a cheapie seat post. The size you want is the internal hole of your frame, i.e the external diameter of the seat post. Be carefull as there are some sizes that are very close together and the wrong one won't work.

I also suggest you remove the quick release for the seatpost clamp and replace it with a normal bolt. This means any would be thief has to come tooled up if they want to do it again and it prevents pranksters/kids from doing it for a laugh!

 

[Pat "5mph"]

Weirdly they also left this mysterious red wax where the seat post used to be

Sorry about your loss
The red wax might be grease, you're supposed to grease your seat post before, erm, inserting it in the frame

 

[Feastie]

Thanks guys, very helpful! I'm sure you're right and that is what the red wax was.. guess I've never taken the saddle off before! Unfortunately my bike is some weird size of seat post and hard to find online any cheaper than in a shop, so I ended up going to Evans and just forking out to get it done - beans on toast for a while! Probably a good thing though as they had to cut it down to size with a metal saw which I certainly could not have done myself.

Many thanks to Cyclist33 for offering me a spare one in exchange for postage! Wrong size but the thought was amazing.