Prices are really only what you will pay for them but here's the common prices items go for:
Black umbrella: 18-19.5k
Jeni umbrella: 20-21k
Pink umbrella: 60-65k
Blue umbrella: 70-75k
Green umbrella: 45-50k
Red umbrella:85-90k
White umbrella:120-125k
Gold umbrella: 300-350k
Rainbow umbrella: 750-800k
Panda hammer:150-250k
Cat melee: 30-40k
Giant Loli(rare): 35-45k
Chainsaw: 12-15k
Crayon:10k (shame, considering how few there are in game)
La chancla: 6-7k
Ice blade: 6.5-7.5k
Panda hug: 4-5k (under 15 in game)
Pink sapphire dagger: 4k
Spear trash pic: 4-6k
Paintbrush:5k
Cigar:3.5-4k
Syringe:3-3.5k
Green drag swords: 27-28k
Red drag swords: 30-35k
Skate tp: 18-19k
Rainbow skate tp: 27-29k
Champlain dagger: 28-32k
Special fb axe: 60-80k (exactly 10 in game, with a lot in active)
Mushroom dagger:priceless (only timo has it)
Kenshi katana: 120k ( prices will likely drop as more are auctioned)
Modern katana: 750-1mil
Layerd sword: 75-100k
Layerd dagger: 1mil
Graalians hammer: 7k
Superfart:2k
Spdfart:2-2.5k
Toy bat: 1k
Skull juggle: 1-1.5k
Spain flag: 8-9k (less then 8 in game)
Solid France flag: worth 1 mil (2 in game, timo doesn't sell his and the other owner quit)
Shiny France flag:2.5-3.5k
Germany flag:2k
Heart axe: 2.5k-3.5k
Please tell me if I have forgot anything major and I'll add it in :) hope I helped!