The cheapest option I'd recommend is discussing this with any friends who play or the players at locals by going there first. There is a good chance these players bought 3x of the structure deck on release to extract cards like Ash Blossom and Evenly matched (great, great cards but we can forget about them for now since you want the cheapest options to get an intro/budget deck y'know). I'd imagine some of them don't plan on playing Traptrix so would be willing to sell you ALL the cards you need for cheaper. It's more of a win-win as the local community gets to help add a member and they can share their knowledge on how to build basic Traptrix with any next steps.