FireLance
Legend
Right, here's something a little more useful for the purpose of this discussion. I've gone through the Magic Item Compendium and pulled out the item levels for all the metamagic rods.
Lesser Substitution (2,700 gp, 7th)
Lesser Enlarge (3,000 gp, 7th)
Lesser Extend (3,000 gp, 7th)
Lesser Sculpting (3,000 gp, 7th)
Lesser Silent (3,000 gp, 7th)
Lesser Empower (9,000 gp, 12th)
Lesser Reach (9,000 12th)
Substitution (10,500 gp, 13th)
Enlarge (11,000 gp, 13th)
Extend (11,000 gp, 13th)
Sculpting (11,000 gp, 13th)
Silent (11,000 gp, 13th)
Lesser Chaining (14,000 gp, 14th)
Lesser Maximize (14,000 gp, 14th)
Greater Substitution (24,300 gp, 15th)
Greater Enlarge (24,500 gp, 15th)
Greater Extend (24,500 gp, 15th)
Greater Sculpting (24,500 gp, 15th)
Greater Silent (24,500 gp, 15th)
Empower (32,500 gp, 16th)
Reach (32,500 16th)
Lesser Quicken (35,000 gp, 16th)
Chaining (54,000 gp, 18th)
Maximize (54,000 gp, 18th)
Greater Empower (73,000 gp, 19th)
Greater Reach (73,000 19th)
Quicken (75,500 gp, 19th)
Greater Chaining (121,500 gp, 22nd)
Greater Maximize (121,500 gp, 22nd)
Greater Quicken (170,000 gp, 24th)
Going by the item levels, the lesser and normal metamagic rods generally do not allow you to enhance a spell beyond the highest level spell slot that a wizard or cleric of your level would be able to cast. The greater metamagic rods still do break this metacap, but not by much - Substitution, Enlarge, Extend, Sculpting and Silence give a one spell level advantage, and Empower and Reach give a two spell level advantage (mainly because 19th-level wizards and clerics do not gain access to 10th-level spell slots). Chaining, Maximize and Quicken maintain the one spell level advantage if the wizard or cleric takes Improved Spell Capacity with his 21st and 24th level feats.
Of course, many campaigns will not use the magic item level rules. Going by standard wealth levels, and assuming that no single magic item can be worth more than half the standard wealth level, you can get Lesser Empower as early as 7th level (a one spell level advantage), normal Maximize as early as 13th level (a two spell level advantage) and Greater Quicken by 17th level (a four spell level advantage).
Final verdict: lesser and normal metamagic rods are not broken if you use magic item levels, greater metamagic rods are on the powerful side (but most likely still not broken) even if you do use magic item levels, but if you allow a character to invest half his wealth into a single magic item, all bets are off.
Lesser Substitution (2,700 gp, 7th)
Lesser Enlarge (3,000 gp, 7th)
Lesser Extend (3,000 gp, 7th)
Lesser Sculpting (3,000 gp, 7th)
Lesser Silent (3,000 gp, 7th)
Lesser Empower (9,000 gp, 12th)
Lesser Reach (9,000 12th)
Substitution (10,500 gp, 13th)
Enlarge (11,000 gp, 13th)
Extend (11,000 gp, 13th)
Sculpting (11,000 gp, 13th)
Silent (11,000 gp, 13th)
Lesser Chaining (14,000 gp, 14th)
Lesser Maximize (14,000 gp, 14th)
Greater Substitution (24,300 gp, 15th)
Greater Enlarge (24,500 gp, 15th)
Greater Extend (24,500 gp, 15th)
Greater Sculpting (24,500 gp, 15th)
Greater Silent (24,500 gp, 15th)
Empower (32,500 gp, 16th)
Reach (32,500 16th)
Lesser Quicken (35,000 gp, 16th)
Chaining (54,000 gp, 18th)
Maximize (54,000 gp, 18th)
Greater Empower (73,000 gp, 19th)
Greater Reach (73,000 19th)
Quicken (75,500 gp, 19th)
Greater Chaining (121,500 gp, 22nd)
Greater Maximize (121,500 gp, 22nd)
Greater Quicken (170,000 gp, 24th)
Going by the item levels, the lesser and normal metamagic rods generally do not allow you to enhance a spell beyond the highest level spell slot that a wizard or cleric of your level would be able to cast. The greater metamagic rods still do break this metacap, but not by much - Substitution, Enlarge, Extend, Sculpting and Silence give a one spell level advantage, and Empower and Reach give a two spell level advantage (mainly because 19th-level wizards and clerics do not gain access to 10th-level spell slots). Chaining, Maximize and Quicken maintain the one spell level advantage if the wizard or cleric takes Improved Spell Capacity with his 21st and 24th level feats.
Of course, many campaigns will not use the magic item level rules. Going by standard wealth levels, and assuming that no single magic item can be worth more than half the standard wealth level, you can get Lesser Empower as early as 7th level (a one spell level advantage), normal Maximize as early as 13th level (a two spell level advantage) and Greater Quicken by 17th level (a four spell level advantage).
Final verdict: lesser and normal metamagic rods are not broken if you use magic item levels, greater metamagic rods are on the powerful side (but most likely still not broken) even if you do use magic item levels, but if you allow a character to invest half his wealth into a single magic item, all bets are off.