First of all, Marco Polo is still a rarity label which records and releases only repertoire that has not been recorded previously or is not currently available in decent recordings. The focus of Marco Polo also has shifted-when the label was started back in 1982 Marco Polo focused on Romantic, Late Romantic and Post-Romantic composers, the best-known works of relatively unknown composers (for example, Rubinstein), and unknown works of well-known composers (for example, Wagner and Respighi). Back then, Marco Polo was the only dedicated rarity label.
Today, many other labels are digging out rarities because they can no longer make money from standard repertoire. As a result, Marco Polo is now concentrating on repertoire in which it is the market leader (light classical composers, film music, Romantic piano music) while also extending its scope to include more 20th Century classics (Markevitch, Lajtha) and Latin-American music. Some projects, like the complete orchestral works of Glazunov and the Romantic Piano Concerto series, had to be shifted to Naxos as other labels started releasing the same repertoire.
The difference in price has nothing to do with the artists but with the repertoire. In order to break even on a recording, we have to sell six times as many copies on Naxos than on Marco Polo (10 times if a work is in copyright). Therefore, if we feel that rare repertoire may not achieve these substantially higher sales figures, even at the Naxos price, we will release it on Marco Polo.
Also, the focus of Marco Polo is still on the late 19th and early 20th centuries whereas Naxos covers all periods of classical music and, these days, releases quite many world-premiere recordings, in particular of 18th century composers but also Early Music and Baroque repertoire, not to mention our guitar and organ projects. In the not too distant future, Marco Polo may also branch out and cover Baroque and 18th Century repertoire, simply because there is a limit to the size of the Naxos catalogue and to the space allocated to the label by retailers.