According to Real Clear Politics there are 40 Democratic seats that are
safe or not in play and 47 Republican seats that are safe or not in play.
This leaves 13 states in play. The following are the 13 states ranked by
the PredictIt probability that the Democrats win the state.
# D seats  state  prob  
41  WI  92  
42  MN2  84  
43  WV  81  
44  NJ  77  
45  MT  68  
46  FL  57  
47  NV  50  
48  AZ  48  
49  IN  45  
50  MO  37  
51  TN  21  pivot for 51 Democratic seats 
53  TX  20  
53  ND  15  
14 = PredictIt marketbased probability of the Democrats controlling the
Senate.
If the Democrats win Tennessee and all the states above it, they will have 51 seats, counting the 2 independents as Democrats, and thus control the Senate. According to the ranking assumption, the PredictIt market probablility that they control the Senate is thus 21 percent (the probablity they win Tennessee). This is close to the PredictIt marketbased probability of the Democrats controlling the Senate of 14 percent. The market price is thus rougly consistent with the ranking assumption, although a little lower. Data collected from PredictIt on October 16, 2018, 3:00pm EST
According to Real Clear Politics there are 40 Democratic seats that are
safe or not in play and 47 Republican seats that are safe or not in play.
This leaves 13 states in play. The following are the 13 states ranked by
the PredictIt probability that the Democrats win the state.

# D seats  state  prob  
41  WI  89  
42  MN2  87  
43  WV  77  
44  NJ  75  
45  MT  65  
46  IN  54  
47  FL  50  
48  AZ  47  
49  NV  40  
50  MO  37  
51  ND  19  pivot for 51 Democratic seats 
52  TN  18  
53  TX  17  
17 = PredictIt marketbased probability of the Democrats controlling the
Senate.
If the Democrats win North Dakota and all the states above it, they will have 51 seats, counting the 2 independents as Democrats, and thus control the Senate. According to the ranking assumption, the PredictIt market probablility that they control the Senate is thus 19 percent (the probablity they win North Dakota). This is essentially the same as the PredictIt marketbased probability of the Democrats controlling the Senate of 17 percent. The market price is thus consistent with the ranking assumption.
Data collected from PredictIt on September 13, 2018, 2:30pm EST
According to Real Clear Politics there are 40 Democratic seats that are
safe or not in play and 47 Republican seats that are safe or not in play.
This leaves 13 states in play. The following are the 13 states ranked by
the PredictIt probability that the Democrats win the state.

# D seats  state  prob  
41  MN2  85  
42  WI  85  
43  NJ  76  
44  WV  74  
45  MT  67  
46  NV  62  
47  IN  50  
48  AZ  50  
49  MO  47  
50  TN  45  
51  FL  44  pivot for 51 Democratic seats 
52  TX  39  
53  ND  38  
29 = PredictIt marketbased probability of the Democrats controlling the
Senate.
If the Democrats win Florida and all the states above it, they will have 51 seats, counting the 2 independents as Democrats, and thus control the Senate. According to the ranking assumption, the PredictIt market probablility that they control the Senate is thus 44 percent (the probablity they win Florida). This is higher than the PredictIt marketbased probability of the Democrats controlling the Senate of 29 percent. The market is not quite using the ranking assumption in determining this probability. It probably is reflecting the view that even if the Democrats take Florida they could possbily lose some states above it. 