What is the cheapest way to make your forehead smaller?

If you are considering cosmetic surgery, it is best to see a doctor in person for an examination so you can better understand realistic outcomes after your doctor has a chance to listen to your exact goals.
Everyone's goals and expectations are different and seeing a photo cannot determine if you would be a candidate.
All surgeries (no matter how minor) has risks.  It is for you and your doctor to discuss these risks versus the benefits.

When you are talking about facial surgery, I do not recommend shopping on price. Look for a surgeon with experience in both facial plastic surgery and hair restoration surgery for hairline lowering. Most female patients who want to lower the hairline choose to do so through forehead reduction (hairline advancement) surgery which results in an immediately lower hairline. The downside is that it takes time for the scar to mature and fade, but the upside is that most patients are satisfied with a single-stage quick procedure which produces excellent cosmetic results without waiting 1-2 years for a series of hair transplants to mature.Be sure to see an expert.All the best,

Hello and thanks for your question- 
The best way to decrease the size of the forehead is through a surgical procedure which advanced the hair bearing scalp down onto the forehead. This effectively lowers your hairline. It's very effective as even a minor change can make a big difference in the look of your forehead. 
As far as cost, please be careful shopping cost- it's your face, you only have one of those and it's what everyone sees and how you interact with the world... I just want to caution you on that. 
Hope this helps and God Bless!
Dr. Robb