Different keys in json objects

Question

I have json response like this and server returns different keys for "tiered_prices"

  "tiered_prices": {
    "100": "23500",
    "200": "23300"
} 
  "tiered_prices": {
    "500": "23500",
    "1000": "23300"
}

When tiered prices values were only "100" and "200" I decoded json response like this.

struct TieredPriceResponse: Decodable {
     let tiered_prices: TieredPrices }

struct TieredPrices: Decodable {

     let firstPrice: String
     let secondPrice: String


enum CodingKeys: String, CodingKey {
      case firstPrice = "500"
      case secondPrice = "1000"
}

}

How properly decode such json objects, where keys will always be different? I tried this answer Swift json dynamic key parsing for json but got error

struct TieredPriceResponse: Decodable {
let tiered_prices: TieredPrices
}

struct TieredPrices: Decodable {

var firstPrice: String = ""

enum CodingKeys: String, CodingKey, CaseIterable {
    case firstPrice = "200"
    case tPrice1 = "100"
    
}

init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    if let precisionValue = try container.decodeIfPresent(String.self, forKey: .firstPrice) {
        firstPrice = precisionValue
    }
    if let precisionValue = try container.decodeIfPresent(String.self, forKey: .tPrice1) {
        firstPrice = precisionValue
    }
}

}