Centre Payline	Pays
3 Bars	5000
3 Cherries	1000
3 Plums	200
3 Watermelons	100
3 Oranges	50
3 Lemons	25
Any 2 Cherries	10
Any 1 Cherry	2

Lets assume this is a standard three reel electromechanical slot machine with the following payoff table based on the centre line:

Weight Table
Symbol	Reel1	Reel2	Reel3
Cherry	3	2	1
Blank	2	3	3
Plum	3	2	2
Blank	2	3	3
Orange	4	3	3
Blank	2	3	3
Lemon	4	3	3
Blank	5	5	8
Bar	4	3	1
Blank	5	5	7
Cherry	2	2	1
Blank	2	3	3
Plum	3	2	1
Blank	2	3	3
Watermelon	3	2	2
Blank	2	3	3
Orange	3	2	3
Blank	2	3	3
Lemon	4	3	3
Blank	2	3	3
Total	64	64	64

There seems to be always 22 actual stops on each reel of a slot machine. The following table shows the symbol on each stop as well as the weight.
There are two interesting things to note at this point. First notice that the first reel is weight the most generously and the third is the least. For example the bar has 4 weights on reel 1 and only 1 weight on reel 3. Second notice the high number of blanks directly above and below the bar symbol. This results in a near miss effect.

Total Weight Table
Symbol	Reel1	Reel2	Reel3
Bar	4	3	1
Cherry	5	4	2
Plum	6	4	3
Watermelon	6	5	4
Orange	7	5	6
Lemon	8	6	6
Blank	28	37	42
Total	64	64	64

Most of the symbols occur twice on the reel, and the blank 11 times. The following table shows the total number of weights of each kind of symbol.
Given the two table of weights and the paytable it only takes simple math to calculate the expected return. Following are the specific probabilities of each paying combination. Note that each virtual reel has a total of 64 stops so the total number of possible combinations is 643 = 262,144.

3 Bars: 4*3*1/262,144 = 0.000046
3 Cherries: 5*4*2/262,144 = 0.000153
3 Plums: 6*4*3/262,144 = 0.000275
3 Watermelons: 6*5*4/262,144 = 0.000458
3 Oranges: 7*5*6/262,144 = 0.000801
3 Lemons: 8*6*6/262,144 = 0.001099
2 Cherries: (5*4*62 + 5*60*2 + 59*4*2)/262,144 = 0.008820
1 Cherry: (5*60*62 + 59*4*62 + 59*60*2)/262,144 = 0.153778
The average return of the machine is the dot product of the above probabilities and their respective payoffs:
0.000046*5000 + 0.000153*1000 + 0.000275*200 + 0.000458*100 + 0.000801*50 + 0.001099*25 + 0.008820*10 + 0.153778*2 = 0.94545 .

Thus for every unit played the machine will return back 94.545%.

Courtesy of WizardofOdds.com