If your on a budget, then you can’t go wrong with using one of the online retailers. 20% off doesn’t sound like much but the savings do definitely add up. eBay is a bit of a mixed bag, a lot of resellers and ‘professional’ traders. You can still get the occasional bargain but it’s not what it once was. Start Collecting boxes are also absolute gems as are the occasional big boxes they release (like Looncurse), if you can find somebody to split the set with.
You could also choose to focus on one of the lower cost games instead, such as Warhammer Underworlds or Warcry.