Magnifications of over 300x can very rarely be achieved. I'd say don't spend so much money on something you could use for no more than a few hours a year.
The long eye relief requirement limits your choices at shorter focal lengths (FLs). The ED2's are an option you might like to consider at $79 each from
AOE, with FLs as short as 2.3mm. Then there are the GSO superviews at medium and FLs. At FLs of 20mm or more, Ploessls will also have at 15mm eye relief or more. These are all sensible budget options. If you've got money to burn Televue Radians or Pentax XWs seem to be the most highly praised choices (20mm eye relief).