There’s no such thing as a once-off expense when it comes to marketing. Something like SEO can’t be turned on and off like a light switch. SEO needs to be nurtured and that’s done consistently, even when SEO is losing you money in the short run, when your website is optimized you’ll start seeing the dividends that SEO holds.
Budget your campaigns correctly from the start and most importantly don’t run an SEO campaign if you’re planning on stopping the campaign if they’re not profitable. It takes anywhere from 6 to 12 months to start producing results so keep that in mind when you’re planning your campaign.