Pressurised CO² doesn't need to be filled up every week, more like every 6-9 months, depending on size of CO² cylinder. I know a guy with a 400L heavily planted tank + 3.5kg bottle just refilled his after 8 months, and it cost was $13.
The initial investment is rather sizey, but DIY aint got nothin' on pressurised systems, I can safely say this after trying to faff about for half a year trying to get my DIY running good and then finally giving in and going pressurised.