Since the custom fields are all text (varchar), you'll need to convert to a numeric data type in order to do any math.
Normally, you aggregate things across groupings of some sort. You don't have to, of course, but for illustration purposes I'm going to do it across make and model.
Select Top 1000000
tblAssetCustom.Manufacturer,
tblAssetCustom.Model,
Count(*) AS count,
Sum(Cast(tblAssetCustom.Custom1 As float)) As cost
From
tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Where
tblAssetCustom.State = 1
And IsNumeric(tblAssetCustom.Custom1) = 1
Group By
tblAssetCustom.Manufacturer,
tblAssetCustom.Model
The magic is in the type conversion. Instead of adding
tblAssetCustom.Custom1 to your report, add
Cast(tblAssetCustom.Custom1 As float). Once you've got that, you can apply the aggregate functions to it.