I can't offer specific advice, because everyone's family is a different size and has different needs; however, there are a few basic things to consider before you buy that have helped us in the going-into-9 years of homeschooling.
Different kids, different ways of learning
If you have younger siblings who will be using the same materials, you can probably justify sinking the money into a full curriculum. However, if your children are like night and day, or (as with mine) all have different learning styles and challenges, consider carefully: What is the resale value likely to be like if it's not a good fit for the next child along? Is it a multi-sensory learning curriculum that could be tweaked? How long will you wait between siblings to use it again? (in our case, you're talking 8 years even if it might work for the next one!)
How much room do you have to store your school materials? Some big box prgrams can take up a whole 3-shelf bookshelf or more just by themselves. Does it require big hands-on projects that will need to stay out for days at a time? Are you a type-A who will go insane at this? Are you an extreme Type-B who will never remember to put it away afterwards? Do you even have a place where the projects could stay out, or would you all be eating off your laps because the table's so full? What if (as above) you've decided to hold onto it - will it fill up a closet somewhere and if so, where will you put the stuff that's currently in that closet?
Yes, I know this is a post about budget-friendly homeschooling! But what I really mean is, could you do it for less without sacrificing quality? Our family ask ourselves this in every aspect of our lives. Cooking (once-a-month), cleaning (homemade, mostly, for a lot less), driving (can I combine trips?)...you get the idea. But even if you do all the same things, have you applied this to homeschooling on a cost-per-use basis?
Take the cost of it. Divide by the number of times you will actually use it. That's your cost-per-use. So the seems-like-a-bargain unit study that you pay $10 for and is good for a month....well, as an example, you only use it for one kid, because the next one isn't interested in robots. Then you plan to use it all 20 days, but things come up...field trips, sick days, unexpected guests. You can't re-sell it because it's a pdf or will cost more in shipping that you paid. So let's say you actually use it for 13 days. It costs you 77 cents per day to use...maybe not bad in your book.
But-if you have 3 children and bought a big box program for $400 and taught them all from it, you'd pay only 74 cents per day used (assuming 180 days) and you could resell it for $200 and apply it to your next purchase which would bring the cost of the next year down to 36 cents per day for the next level of the big box program.Or, if you get that next year secondhand for $200, your second year is free! (So your total cost per day over the two years would be halved).
In my case, I use a mix. I have some (very few) programs that I have held onto and used for multiple children. I have other things that I'm willing to buy as a pdf just to save on printing and our limited house space. It's different for everyone, but consider the different factors before you buy.