Objects that you place into your space will earn you a certain amount of Points
Each object you place into your space will earn you a starting-base of 5 points for each object.
Individual objects can award a large amount of bonus points per object, depending on the type of item and how difficult it is to make. Simple items such as signs and benches will only give you a small amount of points, but more difficult objects like Gazebos, topiaries, and potted trees will be worth more points.
Sets created by combining several pieces together in you 12 x 12 space (they don't need to be touching one another) can award you with bonus 100 to 150 points bonus. Typically you will earn more points based on the amount of items in your set and the difficult of obtaining the object. For an example combining a Red, a White, and Blue Pot will give you a Colored Pot Set. A Stone Bench, Pillar, and Fence will give you a Stone Set. A Scarecrow, Weather Vane, Hay Bale, and Well will give you a Farm Set.
Themes are another way to earn extra points. Individual objects that coordinate together in similar styles can earn a number of bonus points towards a theme bonus. For example, the Yellow Bench, Barrels, and Hale Bale are some of the items that belong to the Country theme. If you place enough of the same theme together, you'll earn a theme bonus.
Some items fit into more than one theme, which potentially can earn you multiple theme bonuses. The themes categories are as follows: Animal, Eastern, Chic, Country, Fall, Oriental, Spring, Summer, Unique, Water, Western, and Winter. For an example the Eastern, Country, Oriental, and Western themes you'll need at least 50 points in the theme's category to earn the a 100 bonus (for a total of 150 points).
Take note that if you recycle more than half of the items on your Gardening Space, then you will NOT earn any Gardening Points, regardless of how it preformed the previous time and you will receive an Garden Score of -500 points. Additionally, if you don't have anything in your Gardening Space when judged, you will receive a score of -1000 from the judges.
It seems that the amount of judges that appear are random and who they are are randomly decided. Some judges may be villagers while others are faceless somebodies. You can have anywhere between four to twenty judges depending you did on your last Gardening Show -- the better your score, the better likelihood that more judges will appear at your next show, but the worse you do the less judges will appear at your next show.
The amount of money you receive based on how your Garden scored multiplied by the number of judges that attended your show:
[Total Gardening Points] * [Number of Judges] = [Garden Show Prize $$$]
So the basic formula for calculating the Garden Points is as follows:
Base Object Value = 5 points * [Amount of Items]
Individual Objects Bonus = Sum([Individual Object Point Value] * [Amount of Items])
Set Bonuses = Sum([Set Bonus])
Theme Bonuses = Sum([Individual Theme Bonus] + [Theme Category Bonus])
Total Gardening Point Score = Base Object Value + Individual Object Bonus + Set Bonuses + Theme Bonuses
It seems that if you score over 1000 points, Dunhill will come back telling you that someone wants to buy ALL the items in you gardening space for twice their value.
For every 250 Gardening Points you get you gain Friendship Points with the villagers. The first 250 will get you 300 FP, the next two an additional 200 FP (500 and 700, respectively), the forth 250 point (751 to 1000, for 1000 FP) will get you 300 points. Anything over 1000 Garden Points will give you 1300 FP.