That's Magento's limitation on prices. If you want to increase price maximum, you have to modify tables in where all prices are set. Those tables are:
catalog_product_entity_decimal
catalog_product_index_price
catalog_product_index_price_tmp
catalog_product_index_price_final_tmp
But, also you'll have to modify all quote/order/invoice/creditmemo related tables, which means all tables with prefix sales_*.
What do you have to change? You have to modify every column whose type is decimal(12,2) to something like decimal(20,2).
For example, if you want to do that on catalog_product_entity_decimal table and its column value, you have to execute following SQL query:
ALTER TABLE catalog_product_entity_decimal MODIFY COLUMN `value` decimal(20,4) DEFAULT NULL COMMENT 'Value';
I'm pretty sure that there are a lot of such places.
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