Посмотрите, это то, что вы хотите?
lag_new <- structure(list(ID = c(1L, 1L, 1L, 1L, 2L), Model = structure(c(2L,
2L, 2L, 2L, 1L), .Label = c("Audi TT", "VW Golf"), class = "factor"),
Month = structure(c(1L, 2L, 1L, 2L, 1L), .Label = c("2012-01",
"2012-02"), class = "factor"), Country = structure(c(2L,
2L, 1L, 1L, 2L), .Label = c("FR", "NL"), class = "factor"),
Activations = c(23L, 2L, 8L, 34L, 8L), avg_price = c(5000L,
5500L, 6000L, 7000L, 6900L), Activations_new = c(NA, -21L,
6L, 26L, -26L), avg_price_new = c(NA, 500L, 500L, 1000L,
-100L)), row.names = c(NA, -5L), class = "data.frame")
lag_new$Activations_new <- lag_new$Activations-lag(lag_new$Activations)
lag_new$avg_price_new <- lag_new$avg_price-lag(lag_new$avg_price)