If you're after the cheapest surround sound solution, then perhaps look into one of these type of units...
It's a surround sound DVD player package with all the speakers for a 5.1 setup. You can plug your tv's sound output into it, but as this particular unit is the cheapest thing around there are things to consider:
1) This particular unit doesn't have any digital inputs, so you can't use the optical out from your tv. This probably isn't a deal-breaker for you as this unit has an analogue audio (RCA / phono) input which your tv should (but not for definite) have an analogue output on it. If you're not sure, these connections are just the red and white plugs. Running the optical output from your tv would probably have a slght increase in quality over the analogue RCA route, but if you're on a tight budget then these are the limitations you'll have to accept. Also the optical out on tv's normally don't support a properly decoded surround sound signal like a DVD player definitely would, they are just an alternative wiring solution.
2) The particular unit I've suggested will run analogue surround sound (it has Dolby Pro-Logic II support), and this just uses 2 stereo inputs and turns them into 5.1, or even 7.1 audio output. The unit also has support for digital surround sound in the form of Dolby Digital (DD), but this will only work when watching a DVD (or audio CD) that has been specifically made with a Dolby Digital option (99% of DVDs will run Dolby Digital). Some films support DTS based surround sound, but this unit doesn't support it, but as the difference between DD and DTS isn't much, and again while on a budget, this shouldn't be an issue for you.
3) A unit as "good a value" as this is very limited on input and output connections. In your case, with this unit you'll be able to watch films in full digital surround sound (plus audio cd's in at least analogue mode) and tv in analogue Dolby Pro-Logic surround sound, but that'll be all you can do. If / when you buy more equipment you won't have any spare inputs to add them to this system - that's when you'll either have to start using audio switching / splitting devices, or just make the move to either a basic "seperates" type amp (that'll just manage your audio equipment - i.e. audio signals and your speakers) or get a full AV receiver (that manages video connections as well as audio) - but this is obviously where you have to commit more funds into your setup than you'll possibly want to (going by your budget).
4) Sound quality probably won't be overly impressive, these units are an entry-level surround sound solution - and you just get what you pay for. The 5 "main" speakers (fronts, centre, and surrounds) will probably be "functional" and adequate, but don't expect any major rumbling for the sub, they'll just give the sound some bass that the other speakers won't provide.
I know you already have a DVD player now, but these all-in-one audio solutions normally have a DVD player included, and if you're limited with funds then this may be your only real option.
Good luck with it all.