Hi,
First check you "Price" column data type it should be numeric (Int,Decimal,Float,Double) otherwise it will not show the proper values.
Apart from that keeping dataset in session/ViewState is not a good idea, so you can write data set in file and store it in some local temp path to uniform your file name you can use Session Id.
[Update]
var tt= TempAdsDS.Tables[0].AsEnumerable().Where( p => p.Field<int>("Price")>=Minimum && p.Field<int>("Price")<=Maximum)
Thanks,
Suvabrata