The two cheapy ones that are generally preferred are Aleks (online) and Singapore (workbooks). Both are highly affordable and generally well-liked. I think Aleks even has a free trial month.
We used Singapore and liked it to a point. The reason we left it was because DS7 was tired of arithmetic and not yet ready to memorize times tables. I think we'll come back eventually, at least to help supplement the more problem-solving/less arithmetical approach that I hope to take with him next year.
BTW, if you go with Singapore, I recommend the Intensive Practice and Challenging Word Problems workbooks. They're much more interesting and thought-provoking. You can probably skip the regular workbook unless you just want to hand the book to the child and let her teach herself.
If I had to pick only one workbook, I'd choose the IP one, since it has word problems in it as well as other kinds of problems.
I really liked the Home Educator's Manual, too. It was good at giving me ideas for games and ways to teach things that I wouldn't have considered. I highly recommend it, again assuming that you don't want the child to self-teach.
There are other good programs out there, but Aleks and Singapore are two I'd recommend looking at to start. And if you try one and hate it, at least you're not out much money.
