x <- c(0, 1000, 50000)

df <- data.frame(ID = seq(1:8), z = c("A", "A", "C", NA, "B", "A", NA, NA))

ID    z
 1  1    A
 2  2    A
 3  3    C
 4  4 <NA>
 5  5    B
 6  6    A
 7  7 <NA>
 8  8 <NA>

ID    z   y
1   1    A   0
2   2    A   0
3   3    C   50000
4   4    NA  NA
5   5    B   1000
6   6    A   0
7   7    NA  NA
8   8    NA  NA

df$y[which(df$z == "A")] <- x[1]
df$y[which(df$z == "B")] <- x[2]
df$y[which(df$z == "C")] <- x[3]

set.seed(84)
df <- data.frame(z = sample(LETTERS[1:11], 15, replace = TRUE))
x <- c(0,1000,50000)

df$y <- ifelse(df$z == "A", x[1], ifelse(df$z == "B", x[2], ifelse(df$z == "C", x[3], NA)))

df
#    z     y
# 1  I    NA
# 2  D    NA
# 3  J    NA
# 4  I    NA
# 5  C 50000
# 6  C 50000
# 7  B  1000
# 8  D    NA
# 9  G    NA
# 10 B  1000
# 11 I    NA
# 12 K    NA
# 13 J    NA
# 14 K    NA
# 15 A     0

df$y <- x[match(df$z, c("A", "B", "C"))]
df
#   ID z     y
# 1  1 A     0
# 2  2 A     0
# 3  3 C 50000
# 4  4 D    NA
# 5  5 B  1000
# 6  6 A     0
# 7  7 H    NA
# 8  8 E    NA

df$y <- x[as.numeric(df$z)]

df$y <- x[as.numeric(factor(df$z))]
  head(df)
  #  ID z     y
  #1  1 C 50000
  #2  2 B  1000
  #3  3 H    NA
  #4  4 F    NA
  #5  5 G    NA
  #6  6 K    NA

set.seed(24)
df <- data.frame(ID=1:40, z=sample(c(NA, LETTERS[1:11]), 40, replace=TRUE))