Everytime we post articles on high PR websites, which might not be related to our niche, we tend to see an increase in our SERP within a few days. The only thing here is that you must keep them coming on a regular basis, and not as a one-off editorial. I personally suggest at least a plan spread over 3 months to see the full effects.
In case your budget is not sufficient, you might as well wait until you have the money, and then start the campaign.