Luiz; the Marklin, Geicha, Lineiol, Bub and Hausser were the best of the prewar toy artillery pieces and vehicles. The scale was typically 70mm or 1/25. Marklin made some diecast 1/45 scale pieces to go with their O gauge model trains. Most were Tinplate Toys, only Marklin and Lineol made solid cast iron and machined brass pieces with elevation and azimuth adjustment gears. Most had firing mechanisms that would detonate a cap and propel a shell some distance; some models even had explosive shells that would explode on impact.
I recently purchased a very comprehensive book on Marklin Cannon from German Amazon; if you post a photo of your cannon I can provide the model number and when it was made. The brass barrel cannon usually have the Marklin Coat of Arms stamped into tha barrel near the breach. I agree they are expensive. I consider them to be functional sculpture. Decent examples turn up on Ebay at reasonable prices.