Actual cost was higher, as the changes required more design engineering*, production line changes, and so on. Plus, that work would have taken time, delaying delivery... whereas accepting a "slight" performance penalty meant getting aircraft sooner (the Australians and Canadians were the first foreign customers**, and wanted delivery not long after the USN & USMC started getting theirs), paying less per aircraft, and getting that "operating cost" benefit of being identical with the much more numerous American variants thus gaining mass-production efficiencies.
Note the highlighted lines below in the quotes from the Wiki articles on the two competitions.
By the time other nations seriously considered the F-18 type, it was the early 1990s and the F-18L was long dead.
* An example of the effects a seemingly- insignificant change can have: One of the original differences between the Australian and US Navy's standard F/A-18 was that the nose-wheel tie bar for catapult launch was removed - the RAAF's Hornets were later re-fitted with a dummy version to remove nose wheel shimmy.
** RAAF: Work on the Mirage replacement program resumed in 1975, and the Tactical Fighter Project Office was established in 1976 to manage the process of selecting the RAAF's next fighter. By March 1977 the office had chosen to focus on the F-15, Tornado, Mirage 2000, and F-16 as well as the F-18A and F-18L. Wing Commander (and later Air Vice-Marshal) Bob Richardson test-flew a YF-17 that was being used as a demonstrator for the F-18L in mid-1979, and was impressed by its capabilities. No F-18Ls had been ordered at this time, and the RAAF did not want to take on the risk of being the lead customer for the design. The Government announced on 20 October 1981 that 75 F/A-18s would be ordered.
CF: In 1977, the Canadian government identified the need to replace the NATO-assigned CF-104, the CF-101, and the CF-116. Subsequently, the government proceeded with the New Fighter Aircraft (NFA) competition, with a purchase budget of around C$2.4 billion to purchase 130–150 of the winner of the competition. Candidates included the F-14, F-15, F-16, Tornado, and Mirage F1 (later replaced by the Mirage 2000), plus the F/A-18 and the F-18L. By 1978, the New Fighter Aircraft competitors were short-listed to just three aircraft types: the F-16 and the two F-18 offerings. Northrop, the primary contractor for the F-18L version, had not built the aircraft by the time of the NFA program, waiting on successful contracts before doing so. While Northrop offered the best industrial offset package, it would only "pay off" if other F-18L orders were forthcoming, something the Department of National Defence (DND) was not willing to bet on. In 1980, the F/A-18 was selected as the winner of the New Fighter Aircraft Project competition and awarded a production order; deliveries of the CF-18 to the Canadian Forces began in 1982.
So basically the F-18L failed because Northrop was unwilling to pay for (and do) the engineering necessary to get the F-18L production-ready without having contracts in-hand... and no one was willing to order it unless it was production-ready.