3061 Error Message On Form

I tried to get help elsewhere with this but it didn't quite work out, so I thought I'd pick a few other brains. So here's the problem from the beginning.

I made a table with three fields:

Field Name----Data Type
Category------Text
Item----------Text
Cost----------Number

I set up a form with two combo boxes and two text boxes. I want to pick an Item from the combo box, and have the Cost appear in the corresponding text box. I set up the combo box to show each field when I click on the arrow.

I was given the following code to put in the After Update section under the Event tab for the combo box:

Private Sub Combo8_AfterUpdate()
Me.Text2 = Me.Combo8.Column(2)
End Sub

That works. I pick an item from the combo box, and its value appears in the text box next to it. Ultimately, I want to edit the value in the text box and have it update the table. I was given the following code for the text box:

Private Sub MyTextbox_AfterUpdate()
Dim strSQL As String
strSQL = "UPDATE MyTable " & _
"SET Myfield = " & Me.MyTextbox & _
" WHERE Id = " & Me.MyCombo
CurrentDb.Execute strSQL, dbFailOnError
End Sub

So far, that hasn't worked. I believe I've made the correct substitutions:

Private Sub Text2_AfterUpdate()
Dim strSQL As String
strSQL = "UPDATE [Primary Table] " & _
"SET Cost = " & Me.Text2 & _
" WHERE Id = " & Me.Combo8
CurrentDb.Execute strSQL, dbFailOnError
End Sub

When I type a different value into the text box and hit enter, I get the following error:

Run-time error '3061':
Too few parameters. Expected 2.

I was given a different line of code in the event that I'm "trying to pass text":

" WHERE Id = '" & Me.Combo8 & "'"

When I use that, the error is the same except it says:

Too few parameters. Expected 1.

Does anyone know the correct code?