library(dplyr)

effective.costs <- data.frame(
  user = c(rep("User1", 3), rep("User2", 3)),
  month = c(rep(c("2019-01", "2019-02", "2019-03"), 2)),
  current_subscription = c(rep("subscription_1", 3), rep("subscription_2", 3)),
  costs = c(70, 20, 50, 150, 130, 170)
)

predicted.costs <- data.frame(
  user = c(rep("User1", 9), rep("User2", 9)),
  month = c(rep("2019-01",3), rep("2019-02", 3), rep("2019-03", 3)),
  subscription = c(rep(c("subscription_1", "subscription_2", "subscription_3"), 6)),
  calculated_costs = c(
    c(70, 50, 110, 20, 50, 70, 50, 80, 120),
    c(190, 150, 110, 210, 130, 110, 250, 170, 110)
    )
)

comparison <- merge(effective.costs, predicted.costs, by = c("user", "month"))

getRecommendation <- function(x) {
  subscription <- predicted.costs %>%
    filter(
      calculated_costs < x['costs'] &
      user == x['user'] &
      month == x['month']
    ) %>%
    arrange(calculated_costs) %>%
    select(subscription)
  subscription <- ifelse(
    length(subscription) > 0,
    as.character(subscription[1, 1]),
    NA
  )
  # I know return is not needed, but I'm used to it... :-)
  return(subscription)
}

effective.costs$recommendation <- apply(effective.costs, 1, getRecommendation)

View(effective.costs)

getRecommendation <- function(x) {
  subscription <- predicted.costs %>%
    filter(
      calculated_costs < x['costs'] &
      user == x['user'] &
      month == x['month']
    ) %>%
    arrange(calculated_costs) %>%
    select(subscription)
  subscription <- ifelse(
    length(subscription) > 0,
    as.character(subscription[1, 1]),
    NA
  )
  # I know return is not needed, but I'm used to it... :-)
  return(subscription)
}

effective.costs$recommendation <- apply(effective.costs, 1, getRecommendation)