Well there really isn’t any liquor that is an exact substitute. All of the orange flavored liqueurs currently on the market have their own proprietary blend of ingredients that gives each product its distinct taste. The best advice that I can give you would be to either find cocktails that don’t require it as an ingredient or try buying smaller bottles of Grand Marnier, as they are available in some markets. Cointreau and high-quality brands of triple sec that use a brandy base are one option, but they’ll cost you roughly the same as Marnier. Van der Hum, an orange-flavored spirit from South Africa, is another option, although this is difficult to locate in some markets and may very well cost you as much as the more expensive options.
So you’re best options are to either settle for smaller bottles of Grand Marnier or try and shop around for the best price that you can get. Triple sec, preferably brandy based, is another option, although these will likely not meet your desire for a 40% abv spirit. They also can vary widely in terms of both quality and strength of flavor.
The bottom line here is that any orange-based liqueur with 40% alcohol by volume will probably cost you at least as much as Grand Marnier,if not more. When you take into account the various ingredients needed for flavoring this spirit, the price is understandable high. If you are not willing to accept the few cheaper substitutes that are available, you’re left with few to no options.