The syns for the sauce sound about right - as it is working out the fat from the milk and the cheese, If you are making a roux, as long as you are using the quantities permittable for healhty A's then you could use these - but is it really worth using two healthy A's for. I suppose if you are making a two person serving you might get away with counting it as i healthy A. - the only Syns then would be the Fat and the flour, probably about 5 syns.
Hope this helps. - there is a sauce book that you can get at the slimming world class, that might offer a more suitable alternative.