I spent next to nothing for k-2 for my oldest and he is ahead in reading and math. This year we bought some history curriculum (Story of the World) but we could have made our own using online resources, but I had the $ and didn't want to put in the time to sort it out. Those sites already listed seem really awesome! For science we just used the library and some online stuff so far, but we are now starting a science curriculum that only cost me $30 for the workbook and teacher manual. My dd is 6 and really wanted her own workbooks this year so we bought her some really simple cheap math and phonics workbooks as well as some crafty type workbooks (cut and paste) from Rod and Staff- they are a Christian homeschool curriculum company, very Bible based and they are super cheap, I got a whole big bag full of books (I think 15 books?) for under $70 Canadian!! We did buy ds a math curriculum this year because I couldn't keep up with his demand, at this age though I only need to buy the student workbooks cause I don't need a teacher's manual to tell me the answers yet.
We are just getting into spending more $$ on the kids' curriculums, but that's because we have the $ at the moment and I am interested in pursuing that. If we were strapped for cash and I really wanted to make up my own programs, I feel confident I still could. The internet and the library and a good used book store are the best resources -I have gotten TONS of stuff from a used book store in town- atlases, dictionary, kids human anatomy books, science and geography related books, Dr Zuess books which dd loves to read- all for so cheap!!
Mostly until now we have read classics - Tom Sawyer, the Bible, Swiss Family Robinson, Captains Courageous, The Wild, The Black Stallion, Chronicles of Natnia, etc - lots and lots of reading- many times spending hours outside on a blanket under a shady tree just reading. Then we'd branch out from there- have the kids answer ?s for comprehension, have them write a sentence or 2 about the chapter we read- this works on grammar, spelling, sentence structure, handwriting. We pull out a map and find the location that the book is taking place in- do some map activities, etc. We learn some interesting history facts, look at pictures of the place either online or from a library book. We can cover all subjects just by branching out from one used book I have on my bookshelf!