It depends on what you consider cost effective.
I made the decision NOT to invest in Steck, because I felt I'd be continually bidding on eBay (as the ONLY source of Steck available to me in the UK, except maybe Charity shops), only to get outbid at the last moment, or else paying over-the-odds to ensure I do win the item I needed.
I'd like to have a high walled keep too, but if it's a keep you want, you can build one using system-X parts. The towers are all made from small wall pieces which can be stacked, it's just the outer curtain walls that can't be. It's more versatile than people seem to want to admit, it's just those darn stupid connectors that annoy.
If you know how big you want your keep, and exactly what pieces you would need to build it, you could wok out the cost using system-x, and then see if you can do it cheaper with Steck.
I think Steck is still more easily obtained in the US, as the Playmobil-buying public isn't as big, so it may be you can snap up a 3666 castle or soemthign similar, and have all the bits you need for your keep.
The biggest problem is getting a lot of bits you don't need in an auction, but not getting exactly the bits you do, and that's going to be very hit or miss with Steck, as I don't think it can be ordered through DS anymore, if you're one vital piece short of completing your keep.