Because less 30" panels can cut from a pane of LCD glass than a 22". I think the largest panes produced is about 14 feet by 9 feet.
Let's say a 30" panel is 27" long by 17" high for simplicity sake. since most 22" are 16:9 ratio, let's just say they are roughly 18.5" long and 10.5" high. Working out the numbers that means a perfect 14' x 9' pane of LCD glass can product about 36 cut LCD panels for a 30" LCD monitor. That same pane of LCD glass can produce roughly 81 cut LCD panels for 22" LCD monitors.
Based on the above, if the manufacturer want to make about the same amount of money from the same pane of LCD glass for both sizes, a 30" LCD panel will need to sell for 225% more. Therefore, if it cost $100 to produce a 22" LCD panel, then it will cost $225 to produce a 30" LCD panel.
The real world is not perfect and there will always be imperfection in anything produced. Let say on average 12% of the pane of glass has imperfections spread all around the sheet. Let's assume those imperfections are completely dead pixels or have at least one of the 3 primary color sub-pixels do not work. Guess what that means... you will need to cut around to areas to get the 22" or 30" LCD panel.
For simplicity sake just reduce the overall size of the 14' x 9' pane of LCD glass by 12% (12.33' x 9'). Now the production of 30" LCD panels is reduced to 30 and the number of 22" LCD panels is reduced to 72. This 12% of defects has reduced the production of 22" LCD panels by about 12%, but the number of 30" panels produced has dropped by 17%. Now it costs $112 to produce a 22" LCD panel and about $269 to produce a 30" LCD panel; or about 240% of the cost.
Now, consider the fact that nearly all 22" LCD monitors use TN panel technology and all 30" LCD monitors use H-IPS panels. H-IPS panels simply costs more to produce because it is a more complex technology. I don't know the actual production cost, but lets say it 2x more; or 200% of the cost of TN panels. Based on the above numbers with 12% defects, the 30" H-IPS panel costs $538 to manufacture.
Lastly, 30" LCD monitors do not sell as quickly or as many as 22" LCD monitors. Therefore, there is a small markup in price simply for the fact that 30" monitors sell slower than smaller monitors.
Do you follow the logic?